Please post your system specifications, and run a complete, thorough scan of your system with the updated antivirus programs. Also download and run HijackThis from here:
http://www.spywareinfo.com/~merijn/downloads.html
and then save the log and post it back here. This way, we can take a look at what processes are running and tell if you're infected with a virus that may be causing your computer to slow down.
In the meantime, if you're running Windows 2000/XP, hit Ctrl+Shift+Esc and go to the Task Manager, and on the Processes tab, click the CPU label and find out the name of the program that's got the highest CPU value. If its anything other than "System Idle Process" let us know.