Also, do you have hibernate turned on? That will also eat up space based on how much memory you have installed. Also, as someone mentioned, system restore will eat up space every time you install more programs or update installed programs. So, turn that off as well.
To turn off hibernate, open up a command window in admin mode. Then type the following:
powercfg -h off
You'll have to reboot after that. That should free up some more space as this will remove the hibernate file.