This is just a shot in the dark, but I had a similar problem with a Yahoo Messenger crash log. My entire 80GB hard drive was consumed by this one massive file! I don't know where this file is stored and can't verify because that was on my work computer.
However, you can see what files are causing this by seeing what files were last modified. View Last Modified Files on Windows Vista - How-To Geek
An excerpt from there:
The simplest method that doesn’t require any additional software is to open up Vista’s Search panel (You can hit the F3 key while Windows Explorer is open).
Change the location to your C: drive or whichever drive you are trying to check, set the Date modified to today, and then check the box for “Include non-indexed, hidden, and system files”.
Press the Search button, and then make sure that the Date modified column is showing up in the list. If that column isn’t showing up, you’ll need to add it by right-clicking on the column area, choosing More and then checking the box for it.
You can then just sort by Date Modified descending to see all the recently changed files, which should give you some insight into what your computer is doing behind your back.
So, first thing in the morning, free up some space, wait a bit, and then do the above. Look at which files were modified, and you should see which files are taking up all the space. Hopefully that tells you something. For me, it was a crash log.