Hmm, do this for me if you don't mind it ?
Check windows update and see how much files were downloaded.
I have a feeling windows is setup automatically to download everything without you telling it so,
Reminds me of windows 8 update issue, let the thing run from right after installation it will try its best to force download updates you dont want.
Check this out also: Defraggler - Download
Just like carnage x suite, but it will tell you whats already in your windows drive.
If you recognize something that can be removed on your own do so, otherwise ask one of us nice brutes here and we'll let you know.
20gb is the minimum size for windows 7 64 bit and that's by Microsoft's standard. That does not leave much room for anything else. Along with all these other very good suggestions, you can remove windows update files to reclaim more space Delete installed updates files from Windows 7