You don't even know what PreFetch is, yet you regularly empty the folder? If you don't know what something does then don't touch it. Ask questions and learn more about it. There is no benefit in emptying the Prefetch folder on a regular basis. Stop reading misinforming "tweaking guides."
This does a good job of covering why there is no benefit to cleaning the folder. If you want a correct description of how exactly PreFetch works, see
here.
Check the following in the registry (Start > run > "regedit") to see if Prefetch is even enabled:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
Make sure
EnablePrefetcher is set to 3.