Reformats sound a lot more "scary" than they really are.
If you computer has a CD/DVD burner, buy some CD-Rs/DVD-Rs and burn everything to them. Alternatively, you could use an external hard drive, or flash drive.
You need a Windows XP disc (and the serial that came with it). You put that in your drive, you restart and right when your computer is booting it should say something like "Press F12 for Boot Options" you press that, and you set the first (primary) boot device to your CD drive. Windows will boot from the disc, and you can begin installation. Make sure you format your drive with NTFS, which effectively deletes everything on the drive.
Realize that you will loose every single program on the machine. Make sure you can reinstall them.
Do you guys think he should use DBAN or a similar wiping device prior to installing XP?