You could use
Eraser on "Erase Unused Space" for individual drives or partitions, which will overwrite any deleted files (you set the number and type of overwrites) and also will reset their file names to a long string of 0's.
There is another free space eraser called
SDelete from SysInternals which is run from the command line.
Drop a copy of the downloaded Sdelete.exe into your Windows folder.
Go to Start > Run > Type 'CMD' (this will bring up a black Command Line window)
Type: Sdelete -p 1 -z C: and press Enter (Example: If you want 1 pass of Drive C:)
The application will start to run.
This sort of program is also useful if for example you want to sell or donate your old computer but still with a working Operating System (assuming you didn't want to give away your Windows CD). You would delete all your files, EMPTY THE RECYCLE BIN, and run a free space erase with however many passes you want. This should make any file recovery operation very hard to achieve, yet your Operating System still functions ok.