Just reformat the hard drive. By CIA standards (or so I hear) a hard drive must be reformatted three times to be officially "clean."
There is a program called Shredder that should be available in the public domain. Let's just say it gets the seal of approval from the 3-letter agencies.
No no no no no. Formatting a hard drives does NOT alter the data. It's all still there, all still accessible by anyone who knows what they're doing. (Like me.:)
You want to WIPE the drive, which is the process of actually overwriting each sector on the hard drive with meaningless "data." Most wipe utilities by default write a "00" to each sector. A single-pass wipe will be fine. There are plenty of freebie wipe programs out there. Go to download.com and do a search on WIPE and see what you find.
MM
There are programs that will remove .....however to be totally safe, you need some of the equipment found in Gov Secure facility that will degauss the drive by using a very concentrated magnetic field...