Also delete your past System Restore & Shadow Copy files as applicable to your OS: XP click here / Win7 click here (expand 'To clean up all files on the computer') / Vista click here.
What antivirus do you use? If AVG, look in the following directory C:\Documents and Settings\All Users\Application Data\avg9\Dumps. I had 500Mb accumulated over a few months, but some, e.g. click here have reported several Gb being present there! It is safe to delete all the files in this directory - I've done with no ill-effects.
Download, install & run SequoiaView click here - it will give you some idea what is hogging all that space. G