HD Free Space in Error?
 

Turned on my laptop and it had a fit. Free space is now approximately 40gb less than it was the last time it was turned on. Any thoughts?

Any clue as to what is taking up the space?

I suggest that you do an 'Error check' on the disk as a starting point. Go to the properties window for the disk (right-click on the drive in an explorer window to get to it). This will do a basic check for errors in the index of the filing system.

There is also a Disk cleanup option in XP, and I guess Vista, that can clear out TMP files etc. But if it was that sudden it sounds more like a FS problem.