A number of effects can slow down a once fast PC. Here are about of the more common.

An overcharged and fragmented hard drive. Whenever your drive is too wide (more than about 80 percent), or too fragmented (the two often go together), it could be slowing down your PC, particularly whenever you don’t have much RAM.

Defragging is the easiest solution, so you should try that first. To defrag in XP, select Start, then My Computer. Right-click your C: drive and select Properties. Click the Tools tab, then Defragment Now. Vista defrags automatically–or at least it does so in theory. See Vista Defrag Problems for details. Or you can go with a third-party defragger. Forum regular Flashorn recommends the free MyDefrag, which looked pretty good when I checked it out.

Whenever your drive is getting full and defragging doesn’t help, you may have to make hard decisions about what you are able to delete or off-loaded to an external drive.