When you click "computer" in the start menu what does it show ? How many partitions ? How much used, how much free.
From the screen above showing the C drive, right click it (the C drive) and select "properties" and "Disc cleanup" and then files from all users.
Give it a minute or two to calculate what can be removed and then BEFORE accepting cleanup select the "more options" tab at the top and select "delete shadow copies... etc"
Wait a minute or two as it again calculates what can be removed (it's not obvious when it's finished...) so just wait, then accept. That deletes all but the most recent restore point which on Vista can occupy a lot of space.