Hey... I've been working with Acidized on this problem for a long time (We're both managers of #Script on irc.gamesnet.net - a 45,000+ person network).
As my origional post says, the file is still in use by a process, if you trace the file to its owning process, you will find that it is infact mIRC. As Acidized said, this is a HUGE memory leak issue and can provide insecurity for the application.