Windows Bootcamp hardrive out of space

I recently used Bootcamp to make a Windows partion, and i have (to my horror) discovered that my hardrive is getting fuller, and fuller even when i am not downloading or installing anything.

The Win7 partion is 44GB, and had around 16GB free space two days ago, but every time i checked there were fewer and fewer GBs of free space.

I have never experienced this on a "real" Windows computer, so maybe it is a Mac/Bootcamp related thing. Anybody know what is taking up my GBs?