There was this persistant, pesky .zip file in my D:\ drive's root directory. Its size was 0 bytes and Vista wouldn't let me delete or move it, no matter what I did.
I searched the intarwebs for possible ways to get rid of the little bastard. Some forum suggested deleting it via a Command Prompt that has to be running with Admin rights.
When this didn't work, I kept looking and ran into a neat little tool called
Unlocker.
Not only did it do the trick, but it also suggested what to do with thel file, which I imagine makes it easy to use even for inexperienced people.
If you ever have a similar problem (including the goddamned "file is in use" message), I suggest trying this nice program =)