If you are running Windows XP, you can decrease the amount of space windows can use for system restore points.

Right-click on "My Computer" and select "properties" from the drop-down menu.
On the window that comes up, there should be a "System Restore" tab.
On that tab, you can click your C: in the list and press the "Settings" button.
Drag the slider to somthing lower than the default of 12%. (It won't let you go lower than 2%)