Hello, I just had and recovered from the same problem. Had an .mkv video from a website that usually has trusted videos, and never had this problem before. The .mkv that couldn't be deleted played fine and everything. Whenever I tried to delete it, it would say "... is in use by ___." ___ being whatever program was trying to delete it. Anyway, I finally deleted it with File Assassin. Never knew that was there. My question is what caused this problem to occur? It did not seem to be a virus or trojan or any sort. I did scans and nothing came up.