A couple of things to try...
Be certain that you have all of the latest virus/anti-spyware/adware definitions installed and be sure to run these anti virus, anti spyware programs daily.
Also...be sure to use the option that these programs provide to do a "complete or detailed sweep", as oppossed to a "quick sweep". This allows the entire hard drive to be checked for potential problems as oppossed to just certain areas on your hard drive.
Also...be sure to empty the "temp files" on a regular basis.
Over time these build up and can cause the computer to slow down greatly.
Be sure to run the built in defragmentation utility that is provided through Windows XP. Just go to :
Start>Programs>Accessories>System Tools>Defrag
Over time the hard drive becomes fragmented and it takes the system longer and longer to access the data needed as your use your computer. Running the defrag utility can help to speed up the system noticably!

Lastly I recommend a (free) program called CCLEANER.
It is a wonderful utility that helps to "clean out" all of the "junk files" that accumulate on your hard drive, are taking up precious room and have a way of slowing your system down.
It can be downloaded from any of the well know shareware/freeware sights here on the internet.
Try www.downloads.com or www.twocows.com.
Both are excellent and highly regarded shareware sites.

OK...hope that helps!
I'm a computer tech and I do this sort of thing for a living!