With 750GBs, I doubt you are low on disk space, uinless you are heavy with downloading movies.etc. It does look, at firsthand, to be a driver problem.
Have a look at Stefan Scherer's posts here, particularly the second one. he seems to have found a driver problem and solution.
http://social.technet.microsoft.com/...9-3aa7be1dbc02
Fwiw.
The previous quote comes from old xp technology. The feature was removed from Vista/Windows 7 during Beta testing.
The Compress Old Files option made running Disk Cleanup so slow it was no longer useful. It also caused problems with the new junction files and brought on instability in the OS.
However, Disk cleanup, as it exists in Windows 7, is still a useful program.