@matrix
Haven't used Windows for extended periods for too long. I know a lot of bloat accumulated in time from updates, but 75GB sounds way too much. Try running Windows' Clean Tool (or whatever it was called).
@alyx I just did. It removed 10GB, but not from the Windows folder. Only 3GB were removed from it.
@matrix
Get WinDirStat to help you see where the bloat is coming from.