NB posts below which point out that this isn't great advice!
Try this article -
Gaining Speed: Empty Prefetch on your XP System
A little known tweak that can help you gain some performance on your XP Professional (and Windows Server 2003) related systems is to periodically empty the prefetch folder. The prefetch folder is used to help speed up the loading of programs; XP will load programs it thinks you need before you ask for them yourself.
Since applications are nothing other than data (files, folders which make up the programs), they are susceptible to the same form of punishment all other data on your system is open to… defragmentation, as well as just residing on your hard disk, if needed, the program must be called from the disk and loaded into memory. If XP (or 2003) tries to prefetch applications you don’t regularly use, then you may be wasting memory. In this article, we look at how to speed up performance by emptying the little known prefetch folder and allowing XP to start the prefetch determination and archiving process over again.
Article continues with info on how to clear down prefetch, and optimise going forward.
RC