Go to this website and download the program and then burn it to a disk. You can boot from it and use it to clean a hard drive. It is a very good program for wiping a hard drive and making all the data unrecoverable.
No one has proved to be able to recover anything that has been overwritten with zeros just one time.
SDelete (sysinternals) is a tool to overwrite single files. It can also wipe out ununsed space on the disk. The problem with secure deleting of a single file is that it might have existed other places in the past. Wiping out unused space might fix that.