If you take yourself over to Download.com,you'll find a number of excellant drive cleaners.I use and would recommend either Eraser or SureDelete.
Both use the top knotch non-recoverable scrubbers that are in effect with the military.
Do a Defragment and Disk Cleanup and then the Disk Cleaner.These programs can take several hours as it takes more than one pass to do the job properly,so set it to do the job overnight.
If this helps-----SS
Is there a good free drive cleaner available - like Evidence Eliminator?