55 gigs gone in a flash!

[private joker]

So I've got an 80-gig HD. As of a couple days ago, I had used 25 of them, and I had 55 gigs free. (I checked this because I recently upgraded my RAM and was checking up on the system). Today I have 0 gigs free -- all 80 are being used. I have no idea how this happened, especially since I haven't downloaded any programs or anything.

I can't even figure out what is taking up this space. Any ideas?