Try disabling Spotlight.
Go into System Preferences, click on Spotlight, then click on the Privacy Tab.
Drag your entire Hard Drive icon straight into the big Privacy Window.
Come back to the forum and post whether or not this QUADRUPLED the speed of your computer!
[edit: a minute later] Some notes: if your speed didn't all of a sudden get lightning fast, then simply remove your hard drive from the privacy window (it's easy -- just click the hard drive icon in the privacy window, then click the "minus" sign below it).
Also, LOTS of people will say *exactly* what you have said, "My compter's gotten slow, I want to remove/delete some stuff to speed it back up again."
This is a total misnomer (wrong word, I'm trying to think of the word that's like "Old Wives' Tale"). But deleting files will NOTTTTTTTTTT (emphasis) speed up your computer. This comes from the old days when hard drives were only big enough to hold one document! One photo, and FULL! And the computer had to do all kinds of antics to work around this totally full disk. Nowadays, things don't work that way.
What causes slowness is your processor trying to keep up with something. And for a Mac's processor to be "trying to be keeping up with something" usually means you got a whole bunch of stuff going on in the background that you don't want and/or don't know about.
That's why we go into Applications/Utilities and fire up the application called, "Activity Monitor." Totally play around with Activity Monitor for a couple hours, and you will have a REAL GOOD idea what is slowing your computer down. Also, while you're in Utilities, fire up "Console." Make sure Console is set to display ALL messages.
This is so important I'm going to repeat it again:
Also, while you're in Utilities, fire up "Console." Make sure Console is set to display ALL messages.
The VERY SECOND you get a spinning Beach Ball, look at Console -- it WILL tell you what is happening.
That's really cool because the old days of "guesswork" are long gone. You can be watching Activity Monitor and Console, and know exactly what is happening, whether you need more ram, or what programs are hogging up resources, etc., etc., and.... who knows, maybe you DID fill up your hard drive with 25,000,000 photos from Disneyland or whatnot. Point is, you'll find out.
But first, simply disable Spotlight and see if that helps! If it does (or doesn't), please post back, and I'll tell you what to do next!
P.S. OMG OMG guess what! guess what! YOUR computer is the SAME as mine!!!!! The 2.4ghz intel core duo mid-2007 iMac! And lemme tell ya, my iMac SAILS!!! It is REALLY, REALLY FAST! I will help you make your computer as fast as mine! Now I'm almost certain it's Spotlight!