Open Activity Monitor in Applications>Utilities, select All Processes & sort on CPU%, any indications there?
How much RAM & free space do you have also, click on the Memory & Disk Usage Tabs.
Open Console in Utilities & see if there are any clues or repeating messages
In the Memory tab, are there a lot of Pageouts?
There are quite a few things that can help. A few of the easy ones are:
First, simply restart the computer. After a long time without a restart, temporary files and caches can pile up, slowing things.
Second, see if your hard drive is getting too full. Right-click your hard drive icon and you get a "contextual menu" like this:
Select "Get Info" as shown in the image. That will give a new window whose top portion looks like this:
The amount "Available" should be at least 15 percent of "Capacity," or 20GB, whichever is larger. In the example, the free (Available) space is about 68 percent.
Third, are you running any commercial ("pay-for") antivirus or internet security software? Those can drag perfromcence down. Dump them.
Fouth, are you using a back-up scheme other than Apple's Time Machine? Those bundled with external hard drives can be real performance killers.
Check those items. If nothing is amiss there, post back and we'll check some stuff that's a little more in-depth.
EDIT: BDAqua out-typed me again! Must put up a "Do not Disturb" sign when working here!
I am not up to the console status. Do not know what to look for in console! For instance I have this message in there as a last entry. What that means?
22/09/12 11:36:50 PM com.apple.launchd.peruser.501 (com.hp.devicemonitor) posix_spawn("/Library/Printers/hp/hpio/HP Device Monitor.app/Contents/MacOS/HP Device Monitor", ...): No such file or directory