You can try this:http://www.dban.org/ or a similar "hard drive wiper" sort of program. These contain a version of the MSDOS within the program, so will run without a working OS. (They have to, since you're clearing the drive completely). You do need to have the BIOS of the affected computer set to "boot from CD", as the 1st boot option. Plan "B" would be to pull the drive, and pop it into another machine, and do a FULL format. If the HDD in question is WD, they have available software to "zero out" a drive, which means writing zeroes to the entire drive. I think "Seagate's "Sea Tools" is similar in this capacity. You might be able to download a copy of any Linux distro, and do a live run of that OS from the CD drive. If you opt to "install" Linux, it will also format the entire drive for you. When you attempt to reinstall Windows, it will then overwrite the entire drive.