reformatting my disk

I am needing to wipe my computer down to all 000000's. How may I go about this? My Dell has had viruses and trojans and worms. I'm wanting to wipe the whole thing clean and start over again before I begin making a serious living off of it handling confidential records. Unfortunately, I messed up my pentium 2, as well. It was programmed with windows 98 and I put in a Dell version of windowsxp and now I want to totally bust the thing down to 0000's, the same as the Dell. The Pentium is working just fine, other than what I did to screw it up. It is foramtted to be programmed with the xp. I just need to buy the xp program. I can probably get away with just removing all the programs off the Pentium and start clean and put in the xp. But the Dell needs a complete overhaul. Too many IT's have told me it's a wash unless I totally bust it down to 0. Where do I find the instructions on wiping out the hard drive and reformatting from scratch? I haven't found it in the library so far. I have the windows xp Bible, but where do I get the step by step instructions on how not to kill the computer when reformatting? Help of any kind WHATSOEVER is so appreciated.