Vista--> Disk cleanup.

Disk Cleanup --> "Files from all users on this computer"--> Continue --> More options (Tab) --> click the "clean up" button in "system restore and Shadow copies" section.--> click "Delete" to confirm deleting --> Click "OK" --> Click "Delete Files" to confirm

Check your disk free space again.... :-)

Vista creates Restore points and shadow copies automatically, and they take a lot space. Do these steps periodically to keep your disk from cumulating a lot of junks.