Q: my mac is about 5 years old and running very slow, what can I do to get it back to it's fast self?

What year, screen size, CPU speed and amount of RAM installed in your iMac?

How full is your Mac's hard drive?

How many applications do you run simultaneously in the background while working in another application?

Do you run any antivirus software on your Mac? Antivirus software can slow down the normal operation of OS X.

Do you run any "crapware" like Mackeeper or any other type of so called hard drive "cleaning" apps?

Get rid or Norton NOW!

Norton Antivirus (made by Symantec) has a very long and illustrious reputation for mangling Mac OS X systems, sometimes to the point where a complete reinstall is necessary. Among other things, it installs kernel extensions which are known to cause kernel panics and system freezes; it contains known and documented bugs which can silently corrupt Adobe Photoshop and Adobe InDesign files, destroy a user's ability to authenticate as an administrator, and (on PPC systems) can cause Classic to stop functioning; and Symantec has on at least two occasions now released flawed .dat file updates which erroneously report certain critical Mac OS X files as "viruses." (Deleting these "viruses" causes damage to the system that in some cases renders it unbootable.)

Norton Removal Tool (Symantec Uninstaller):

http://www.symantec.com/business/support/index?page=content&id=TECH103489&locale =en_US

Yeah,

Gid rid of Norton Antivirus!

This is the worst of the Mac antivirus apps and is known to give a lot of false information about viruses and Trojan detection on the Mac. Plus, it is well know that it can have very detrimental effects and consequences on the OS.

Other things,

You are starting to get low and hard drive space.

Have you emptied your iMac's Trash icon in the Dock?

If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.

If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!

Other things you can do to gain space.

Delete any old or no longer needed emails and/or archive older emails you want to save to disc, Flash drive/s or to ext. hard drive.

Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.

Again, archive to disc, Flash drive or ext. hard drive and/or delete any old documents you no longer use or immediately need.

Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon  and empty the Trash.

Also, if you save old downloaded  .dmg application installer  files, you can either archive and delete these or just delete the ones you think you'll never install, again.

Download an app called OnyX for your version of OS X.

When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.

Typically, iTunes and iPhoto libraries are the biggest users of HD space.

If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc, Flash drive or ext. hard drive and/or deleted off your internal hard drive.

You may have to Purchase an external FireWire or Thunderbolt hard drive to move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.

Also, your iMac model can take up to 6 GBs of RAM.

You can increase your RAM by another 2 GBs.

Purchase and install one 4 GBs RAM module to replace one of the 2 GBs modules in your iMac, now.

Correct and reliable Mac RAM can be purchased from online RAM sources Crucial memory or OWC (macsales.com).

Here is what you need.

http://eshop.macsales.com/item/Other%20World%20Computing/5300DDR2S4GB/

Good Luck!

In addition to the previous advice, see:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

Small chance your HDD is dying (which it is probably not, check with Disk Utility), you have more than enough RAM, you have enough free space on the HDD.

“

Antivirus: Norton

Crapware: None, I know better...

"

GET RID OF NORTON. Like Klaus said.

I wouldn't say 72 GBs of HD space is a  boat load of free HD space.

PEC --

Great.  But do get an External Hard Drive, and off load the music, photos and videos you don't need regularly.

Your 72 GB is approaching low.  You need 15-20% of your HD free.  So store your big memory hogs to an external hd.  It will save you tons of upset & worry.