here is how I typically troubleshoot the same issue on customers machines;
run the following
CCleaner - free program to remove unused system files and clean up registry
avg free virus scanner
adaware spyware removal
spybot spyware removal
after completing all this defrag using a good defragmenting program, such as diskkeeper 8, and defrag at startup for best performance.
if that doesn't work, run memtest86+ to check for memory leaks and errors.
If all that checks out you should try backing up all your files and reinstalling the operating system on a clean partition
BTW, how long has this operating system been running without being reinstalled? system file clutter and things of that sort tend to decrease performance significantly, I recommend a clean install annually.
If that still does not work your processor, ram, or video card may have some memory leaks at which point some basic hardware troubleshooting would be required.
