kraftwerk

I will assume that you see this in XP, right? The most likely reason why the HDD size goes up and down is the swap file size, if the swap file/paging file is set to be controlled by Windows. If you can add an extra HDD (preferably wiped clean, but formatted - does not need to be bigger than 20-40GB) to the system and then set the swap/page file to that particular HDD (thus bypassing the default Windows settings), you might see that the the HDD free space stabilises. Resetting the swap/page file location will require a reboot.

If you see thie problem in Vista, I'm afraid I can't help you straight away. I haven't checked this out, but Vista would seem to take *great* liberties about accessing the HDD as well as RAM, and using any available space for a good many things.