It could be malware trying to protect its payload.

I can’t recall how to restore this using the edit registry, you could try a google search for restore hidden files in registry. http://www.google.com/search?q=restore+hidden+files+in+registry this is just one, http://v2.tlab404.com/articles/detail.asp?iFaq=198&iType=13 and another http://www.kellys-korner-xp.com/xp_tweaks.htm entry 155.