hard drive shows that it's full but it's not really

Hello,

A friend of mine has a strange problem. He just bought a Lenovo 3000 laptop with windows vista on it, and a hard drive of 80 GB.
A few days after he bought the computer, the hard drive started to show that it's more and more full, and after a few days it got completely full. But he didn't put anything new on it during these few days. I looked at the content of the hard drive with spacemonger, and it showed that the disk is really full. But when we calculated how much was space was actually occupied, it was only about 40 GB (10 GB vista stuff, 10 GB music, 20 GB pictures, a little bit of documents). I made some space by disabling hibernation and deleting some pics and music, but after a few days the disk got full again.
I reformatted the hard drive (only one on this computer) and installed everything fresh: thunderbird, firefox, office 2007, endnote, ccleaner, nod32. There was 40 GB free. A few days later and no new additions or installations, the hard drive is full again!!!
Anti virus/spyware scan is clean.
What could be the problem?

Thanks...

 

http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/capable.mspx

here's your system requirements for Vista OS. So once read it appears you would need a total of 55 Gigs just to run the program. Memory suggested is at least 1 Gig. Is this a new computer with Vista installed? They sure didn't leave much in the way of a hard drive to work with. Have you gone onto the Vista site and downloaded the program they have to check your system specs and determine whether it's a good OS for this computer? Maybe someone else knows something more but that is the best I can do. There are other forums here that deal with Microsoft Vista and you might have better luck posting there. Let us know...Jazz