Windows xp uses this feature to load common programs at boot time. A feature sometimes not so efficient.
Clearing prefetch cache is easy. Find prefetch folder in your windows folder and delete all files in the PREFETCH folder. The next boot will be slow but others to come will be faster! After a couple of days/weeks the prefetch folder will be full of programs again.
A little registry hack solves this little problem. Start, Run.., Type "regedit".
Find the key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters". Here you will find a value named "EnablePrefetcher".
Four values are available:
Disabled: Prefetch system is off.
Application: Caches only applications.
Boot: Caches only boot files.
All: Caches both boot and application files.
The drawback with setting the prefetch setting off is that applications will start slower.