It's pretty easy. In fact Microsoft has done a nice job of taking all the pain out of wiping the hard drive and reinstalling XP. Pop your Windows XP disk in the CD ROM drive and wait for the installation menu to come up.

Choose "Install Windows XP" then it will go into a new screen that lists a bunch of steps. From the from the pulldown menu choose "New Installation (Advanced)" and let the software do the rest.

It will wipe out all the data on your hard drive and put a fresh copy of Windows XP on the system.