A couple of pertinent questions - do you have anything on the existing hard drive that you cannot do without? A complete data backup is probably in order before you proceed. Do you have any peripherals that XP's "native" installation media won't have drivers for? Printers, scanners, etc, etc - if they came with drivers on disk you'll need to locate those disks. No network means you won't be downloading them from the internet (on that box, at least.)
Basically you'll need to slick your hard drive and treat XP as a "new" installation. FReepmail me if you don't know precisely how to do that. Do NOT attempt an "upgrade" since that won't work anyway. Slick the drive and start over.