This is a weird. This shouldn't happen on a fast machine. Are you using WinXP? In WinXP, right click on my computer->go to properties->advanced->performance settings->advanced. Then make sure processor scheduling and memory usage are set to 'Programs'.
Also, make sure you are not running in compatibility mode. (right click my computer and check the compatibility tab.)

Spysweeper is a great antispyware, but you can also add ad-aware and spybot to your collection to make sure your system is clean. (make sure they are updated before running a full scan). After running a full scan, make Spybot 'immunize your system'.

A good registry cleaner can also clean computer slowdowns and frequent crashes.

If that still doesn't fix it. I could recommend some more advanced tweaks that may require other programs. I have tons of tweaking stuff/info here in my comp.