--DO THIS AT YOUR OWN RISK--
Start > Search "Backup & Restore Center" > Click "Create a Restore point or Change Settings" on the left > It will load your hard drives, check C:\ (or your default hard drive) hit apply, then check it again, and hit apply.
This will delete all your restore points and should free up quite a bit of Hard Drive Space, it free'd up 20GB for me, one guy lost 300GB to this.
Don't do this if you install lots of things and are a risky person!
(Also works on older versions of Windows but Vista uses the most space)