Check the recycle bin. Remember that there's a separate recycle bin for every account on the system, so check all of them.
System restore points can also consume quite a lot of space. To see how much space is being used, do this:
Start -> right-click "Computer", select "Properties"
Click "System Protection" in the left pane.
Click your "C:" drive under "protection settings" so that it's highlighted, then click the "Configure" button.
You'll see how much space is currently used and you'll also see a slider which you can use to adjust the maximum amount of space allows for restore points.