Hmm... It could be a program automatically downloading files from the internet or something, but i doubt a program would need to download THAT much unless its a virus. It probably would be your hard drive.
well i had a sort of virus that did the same thing as your hdd,
every day i lost GBs then i found the problem in a hidden folder,
all it was is a folder called " _ " and it just kept multiplying , when i caught it, it was up to like 30 gbs
Freestyler has hit the nail on the head I remember that Vista has an automatic system restore function that saves every day or so, so it will b baling up your computer and draining your hdd, you will need to disable it and run a manual back up. I'm back on XP now so cant quite remember where to look but I know its there Ive seen it when I ran ultimate.
well i had a sort of virus that did the same thing as your hdd,
every day i lost GBs then i found the problem in a hidden folder,
all it was is a folder called " _ " and it just kept multiplying , when i caught it, it was up to like 30 gbs