Do You Really Want To Format That Hard Drive?
Be forewarned, formatting a hard drive is MUCH different than formatting a Word document. The format will erase *everything* on the disk (the operating system, your programs and data files) and leave you unable to even boot up the PC. About the only thing you can do with a freshly formatted drive is install an operating system from CDROM.
I know... that's exactly what you said you wanted to do. But that *may* be a bit drastic. After re-installing Windows XP, you will need to reapply all the patches and fixes from Microsoft, then reinstall all the software packages you previously had. It could take many hours just to download the Windows Update files.
I'm guessing you're considering this move because your PC has been infested by a bunch of spyware and/or viruses, and is now acting sluggish and flaky. But before you go all samurai on your hard drive, I recommend you follow all the steps in my Make Windows Run Faster! article. It's my special recipe to clean the gunk out of your computer's pipes, so Windows will start quicker, run more reliably, and go faster on the info-superhighway.
If you really want to format your hard drive...
You can't... at least not while you're running Windows. You'll have to reboot from the Windows XP installation CD, because you can't format the drive on which Windows is running. But first you'll probably want to back up your hard drive, or at least part of it. For help with that, see Backing Up Your Files and explore the various options for backing up your personal files.
Okay, back to reformatting... when the install disk boots up, you