Make a bootable floppy and put the old HD in alone. Boot to the floppy and then use mbr command with Fdisk. Use fdisk to wipe the partition and then recreate it. Format it and the HD will be clean.

You don't HAVE to use the MBR command but to be safe, do it anyways.

And one other thing, make sure the boot floppy is write protected when you use it. If you boot up and something tries to write to it, you've got a virus.