As Igor said, its likely this might be in one of your regularly visited sites, and just keeps re-implanting itself everytime you browse. However, can I make a suggestion? Download this product:
http://www.spazmatic.net/partners/ccleaner/
Yes, its called Crap Cleaner, but its a very nice little freeware program I install on all my machines. Set it up to clean your temp/caches and everything else on reboot automagically removing all of those locked file nonsense and whatnot, and forget about it. But most of all, you can simply “Run it”, clear your caches anytime you like. It even deletes those index.dat files windows makes that everyone hates so much.
Enjoy.