Open Disk Utility (in the Utilities folder in the Applications folder) & select "Macintosh HD" (or whatever you have named the startup volume if you have changed it) from the list on the left.
At the bottom of the window, you will see among other things the "Available" hard drive space. This number should be at least 10 or 20 GB, ideally much larger. If it is not, you need to move some files off the hard drive or delete them. If you have plenty of available space, there is no particular reason to move or delete files, including those for apps you don't use. It is not a good idea to delete the Downloads folder itself, although you may want to delete some items in it if you need more available hard drive space.
While still in Disk Utility, click the "Verify Disk" button on the right. If it finds any problems, you will need to start up from another volume & repair them.
In general, quit applications you are not using. Adding more RAM (memory) will help speed things up.
There are several things you need to look at.
#1 - Find out exactly which model you have (Model ID) : "Apple : About This Mac : More Info : System Report : Hardware : Model Identifier : (should be something like iMac5,1)
Note: You state Late 2006, but you say intelcore duo (if not a core 2 duo, then it's early 2006, this #1 above), depending on the model, you may only be able to get 2GB or 4GB of RAM etc. but at least it will be cheap. There are several places to purchase, I personally use Crucial, but OWC is good too.
# 2 - While using System Information (above), you could click on the "Storage" (under Hardware as well), and it would tell you how much space you've used on your Drive/s etc. You could also just highlight your Hard Drive Icon and "get info" on it (this would show you the info as well) - and as RC said above, you want at least 20GB free. Also, yes, if you need space, backup or delete any junk you don't care about (not in Apps etc.), just your own stuff.
Note: Regardless of the space left, 2006 is a blessing for you and your drive, you really should think about backing up all your stuff, and preparing for the death of the drive (I.E. replace it soon) IMHO (3 years is avg life) - Some get 1 year, some get 5 years or more, but boil it all down and you get 3 years (in my 19+ years of IT).
# 3 - As RC-R stated, open up Disk Utility and Verify, and I would also recomend running a Verify/Repair permissions (if you haven't in a long time) - This really isn't for speed, but I recomend it.
# 4 - Fragmentation (oh boy let the flame wars begin) : While in general (everyday use) OSX takes care of small file frags, it does in fact over time (especially with large files etc.) become fragmented, I'm not saying you should run out and purchase Drive Genius etc. but if you've been running this box for years on the same load of OSX, with just upgrades as you went (I.E. Never a fresh install since 2006), then you may want to think about backing up everything (Time Machine etc.) and doing a fresh install of 10.6 (via DVD). This could be done on your new drive if you choose to buy one, but if you don't, it could very well help (5% to 20% - just depends). Let the flames begin!!!
Ok it's early, and gotta run to the day job.
PS. Once you price RAM, and HDD etc. and your time spent, it may be time to peek at an upcoming Mac Mini or something. I believe in using the heck out of equipment, but that box is getting a little old in the tooth. IMHO.
# 4 - Fragmentation (oh boy let the flame wars begin) ...
There are many opinions on the value of defragmenting your startup drive, ranging from 'totally useless' to 'you gotta do it.'
Personally, I think that if your startup drive has never been filled up to over say 60% to 70% of its total capacity, you probably won't see any appreciable benefits by defragmenting it. That's because OS X tries to avoid fragmenting files in the first place, & that works best when there is plenty of free space for it to work with.
But if it has ever been filled up to more than that (very rough!) ballpark figure you may see a noticeable improvement in system responsiveness. It isn't likely to be a whole lot (20% is pretty optimistic), but it should be noticeable on some things.
As for personally, it's for sure a personal thing, depends on the user and their habits etc. Again, the main daily user will more than likely never need to worry.....but, I'm not sure what the users habits are, thus I thought I would bring it up as a last thing to at least just check on.
Example: I always, always, divide my HDD's into at least 2 partitions, 1 for download/encoding etc. and possibly one for imaging/VM's etc. I usually get my downloads/encoding/scratch cranked to 45-59% percent fragmented every 2-3 months, and yep, performance hit happens. OSX HFS+ just doesn't keep up with it. Things have probably changed under the hood between 10.1 through 10.6 and beyond, but even archived discussions with Apple even talk about how it becomes fragmented (depending on the situation). However, along with this, my OS drive/partitions never need defragged (or if so, it's been 2+ years of daily use), as they are only holding things that are pretty stagnent, and when deleted, they are in that under 20MB range in the first place, and also, I usually have TONS of room left (like you mentioned, not getting close to full helps due to OS/FS intelligence of allocating data).
I would say (as mentioned) if it's the same install since he got the iMac (with just updates) and never a fresh install over 6 years, and working with CSx (not sure what he does with it) it might, just might, be worth looking at.
Anyway, just something to chew on.
Just to be clear about it, my comments were about the startup drive (actually, it is a volume or partition). That's the partition the system is on & normally supporting files like the VM store, user home folders, etc.
That is the only partition that OS X aggressively attempts to keep free of fragmentation. Other partitions & drives don't get this same benefit. That is one of the reasons OS X works best if you don't store user files, VM, etc. on other partitions.
100% understood, been working/supporting Mac's since OS 6.x (at one point I supported just over 4k of them). Again the main reason I divide them up is due to the OS's inability to handle and without fragmenting over time huge files (4 to 30+GB) especially when close to full then moving/delete off to storage etc, and it also makes my defrags much quicker as it's only the dedicated partition, or second/third drives in Mac Pro's vs. the 1. Now with that being said, it's been several iterations of OSX since I've placed everything in one volume/partition, maybe I'll give it a whirl on the next format/reload of my older Mini when Mavericks surfaces and give it a good thrashing. I/O performance tests on fresh load and then after becoming fragmented.....ummm...used a little.
Apple has even archived their info on Defrag (but still good info) as the OS/FS has matured so well, but I guess my old habits die hard (especially when they still work for me), course the approach/tech of their FS is to target 95% of users, as I think they have done so very well.
Note: On my Macbook Pro (and wife's) I only use the 1 volume/part, as I don't produce the same type of size/movement, but again, that's just my situation.
However, I've recently built up a new 8 Core , 32GB RAM ESXi mini server for the house, so I plan on pushing all that heavy movement over to it.
RC-R - Thankfully, we're having a good conversation, and in fact I think I will give the old mini a 1 partition trial soon (haven't used 1 part since 10.2?)
zsara t - Any further info? - Note: Please take into account what RC-R and I have been saying, #4 (Defrag info) could be useless info to you, so take it with a grain of salt. However, if you (or anyone else) is heavier user and think you might benefit, may I suggest Prosoft Drive Genius, you can read reviews on the Apple Store, and if you visit the customer site, you can download a demo, which is very limited, but you can at least scan the drive to see what it reports. However, with the specs you presented, and the age of the drive, again, if your model supports only 2GB, I'm sorry to say, it's time to think of upgrading and maybe dedicate your old to just web surfing/ general use for the house? If it can support 4GB, and you can change the HDD yourself (or if a local Apple place will do it for cheap), I would do so and squeeze more use out of it. Keep this in mind though, a Mac Mini (example) on tax free day with student, school worker, state or federal worker discount to boot (if applicable) and free shipping becomes more attractive as it sinks in, and compared to the specs of your old machine, you could easily get away with the entry model. Course I would wait just a little bit on the upgraded CPU's.
I usually advise users not to partition the startup drive unless they have a very good reason to do so.
One of the reasons I have for this advice is partitioning it reduces the available space available on the startup partition, which makes file fragmentation more likely as that partition fills up. Another is that fragmentation has to be quite severe for it to have much of a noticeable effect on systems using contemporary ATA/SATA drives, since their adaptive buffers, out-of-order R/W, & other "smart" capabilities reduce the number of seeks necessary to string together the fragments.
And of course, in the real world the system may be reading from & writing to many different files concurrently, so sometimes partitioning & spreading the files over the partitions increases average seek distances because the heads have to move between the partitions so frequently. That actually decreases the responsiveness of the system!
Anyway, it isn't a "one size fits all" kind of thing. Whatever works best for each user is all that matters.
Besides the discussion here of other matters based on many hypotheticals, what is most important is that you do not have enough RAM. 1 GB, while it may satisfy the System Requirements for running Snow Leopard, is absolutely bare bones, and as soon as you have more than one or two applications open and running concurrently things will be slow.
You need to max out your RAM and/or run fewer applications concurrently.
If you have at least 10GB free drive space, while it may be advisable to free up more, according to what you know about your own usage pattern, that is unlikely to be the cause of your slow performance.
You should also look in Activity Monitor in /Applications/Utilities, go to the CPU tab and see how much CPU is idle. Is there any application or process using an inordinate amount of CPU?
I agree bud, I never tell my friends (except one very high use/huge file engineer) to use parts either (nor the poster here), and I personally try to keep my startup part with at least 50-75+GB free ususllly (on my small box), and again, my (just me personally) speed is great, as the extra volumes do not get much reads and rights during the normal use of my system, specific use etc. and my main system vol/part stays very clean and smooth iops.
Good advice and info for sure!