The Prefetch folder in your Windows directory is used to store information on the applications that you always open. This is to speed up the loading time of the applications the next time you open them. It is not advisable to delete the contents of this folder frequently as it will need time to collect the information again. Deleting the contents in the Prefetch folder about once a month should be alright. Once it is empty it will normally take around 3 boot-up's before Windows will have collected enough information for the applications that you run regularly.
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