I am sorry, but I am inclined to disagree...

"Generally people also recommend emptying the Prefetch directory once in a while. But Windows uses this directory to speed up launching applications. It analyzes the files you use during startup and the applications you launch, and it creates an index to where those files and applications are located on your hard disk. Using this index, Windows can launch files and applications faster. Utilities like CCleaner too have an option to clear the prefetcher. Should you choose to use this option of 'clearing prefetcher', be ready to run an 'un-optimized' windows for a little while. The Prefetcher is best left alone ! In any case, Windows cleans it at 128 entries down to the 32 most used application's prefetch files.

Every time you clean up the Prefetch folder, you delay application load times, the next time you launch them. Its only after the second time that you regain optimal application load times. Only one Prefetch file is created per application .  In Vista this folder does not occupy more than around 50MB. Cleaning the Prefetcher can therefore be construed actually as a...a temporary self-inflicted un-optimization ... "

From Avoid Tweaks of Dubious Value.