What iMac, what OS, how much RAM & how much disk?
Titainium Onyx is also another good free clean-up utility. Also check the amount of free disk space... if it is close to full (less than 5 GB, or has been run right up against the limit, you are running out of swap space (also check to see how many aps are running). iDefrag (not free to do anything but look) is invaluable to defrag.
dave