Open "My Computer" from your desktop. Then right click drive "C" and choose "Wipe free space".

You will then be prompted to enter the number of passes etc.
For a secure wipe choose "U.S. DoD seven pass..." You can choose "User defined..." and set it for even more passes. Be sure and check Wipe "swap file", "file slack", and, "empty directory entries".

Open "My Computer" from your desktop. Then right click drive "C" and choose "Erase unused space".

You will then be promted with the "Confirm" box.
Befor you select "Yes" click onto "Options" to set the number of passes.

Choose at least 7 passes (U.S. DoD) for secure deletion.
Be sure and check the three "Overwrite" boxes below.