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I use it all the time for Temporary stuff like Temporary Internet Files, Cookies, History, TEMP files. It gives the Internet a little extra zip when using 56K.

You can delete everything in your Prefetch folder without causing any harm, I do on a regular basis. Some people have told of losing their Prefetch folder completely after installing SP1.

The Registry is stored in a group of files in your C:\Windows\System32\Config folder (assuming that Windows XP is installed on C . The files with no extension (Default, Software, System, Sam, Security, Userdiff) contain the actual Registry entries. There must be some software out there that can edit these files.