Only if all else fails, you could try to low level format the HDD. That completely wipes it. Writes zeros to EVERYTHING. Catch is how do you get a "Low Level Format" program ? They run from a floppy, so you need a floppy drive to do it. If you have access to another computer, you can download the appropriate prgram from your HDD manufacturers site, or use TESTHDD, which you can download Here :
http://www.testhdd.narod.ru/
What you do is click the underlined download link in the upper left corner of the page and that will download "TESTHDD". (The site is in Russian, but the program is in English). Once you have it downloaded, you take a floppy and make it bootable by right clicking the A Drive then left click "Format", then in the popup window, checkmark the "Create An MS DOS Startup Disk" box, then click "Start". That will create a bootable floppy disk which you copy the downloaded "TESTHDD" program onto.
Once that is done, you enter the bios on your computer and set "A" drive as the first boot drive. Having done that You can then use that floppy disk to either : Format your "C" drive, or FDisk your HDD, - wiping all partitions on it, or "Low Level Format" your HDD, writing zeros to EVERYTHING on the HDD.
NOTE : Formatting is preferable to Fdisking, and Fdisking is preferable to Low Level Formatting.