I forgot: Check "Recovery" in your system and check how much space is used for that.
- Control Panel
- Recovery
- Configure system restore
- Small windows opens and here you choose: Configure
- Press on it and here you will see how much "Disk Space usage" this tool is using on your system. You can reduce that by telling the "Max Usage" how many % of the available space are allowed to use from that tool. Do not make it zero, as if you have a problem, the probably last clean installation on which the computers recovers will most likely be gone.
You also can download this tool:
http://windirstat.info
Run it and you will see how much space is used for what.