Yeah, the brand of the laptop would also help.
I use a Lenovo Thinkpad and had the same problem. Turns out, the thinkpad lines had the back up program turned on by default. Google your laptop brand and see if you can find out what your back up program is called. Then open up the program and see if you can't disable it or limit the amount of space it takes up.
It's an ACER aspire 5633... and runs on Vista. It has it's own 'Acer' backup, but this is non-active. Norton does the back ups.
Could it be something to do with file management? I've also tried resizeing the shadow storage space, but to no avail....