Windows XP monitors the files that are used when the computer starts and when you launch applications. By monitoring these files, Windows XP can prefetch. The prefetching data is the process by which data should be requested is read ahead in the cache. Prefetching boot files and applications reduces the time needed to boot Windows XP and start applications.
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Cleaning the prefetch folder is a myth, the folder is self cleaning by Windows. Cleaning it will REDUCE performance.
http://home.comcast.net/~SupportCD/XPMyths.html