|It could be a virus. There are a number of them designed to fill up a hard drive with useless junk. |
It could also be the size of your restore point or temp internet files.
If you used the same software on the desktop and the laptop and are experiencing the same problem, I would lean towards doing a complete anti virus scan and also check any floppys and cds you have.
You might try Symantecs site, and use their virus/bug scanner or look for (google housecall.
Any installed anti V programs already installed may have been disabled by the virus and report your system clean when in fact it's not.