How to remove all files before giving computer to someone else

I would like to wipe clean my old computer of all files and any information yet still have it be useful to my son. Is there a program I can buy or better yet a free download to help me?

 

Just pop in any OS media and format the hard drive. Don't actually install anything, just format it.

 

I cannot find the original disk that came with the computer, now what?

 

Now I say you try Google or click this link.

http://support.microsoft.com/kb/255867

 

Thanks for your time, I will give it a try