How can I stop something (a worm?) from "eating" my hard drive space?

One day my HDD (pc: Athlon 1900+, 40GB, 512 MB RAM, Windows XP) was full even though there ought to be plenty of free space. I tried to delete some files, and had a few hundred MB of free hard drive space, so the system would operate again, but within less than a minute the HDD was full again. I have tried deleting files several times now, but the same thing happens again and again. I don't know, if it is a worm, a virus or something else "eating up" the space, but the problem is that - because there is no free hard drive space - I can't run AVAST antivirus or SpywareDoctor - or any other program for that matter. What can I do to solve the problem? I have about 2000 family pics on the HDD, so I really need your help. Thanks.

Sincerely
Jens, Denmark