Assuming you have the XP install CD then do a re-install of XP and during the install choose to FORMAT the hard drive.
This wipes everying off the hard drive, and then does a new install of XP on the disk.
Note you have to also give him the XP install CD, you cannot keep it as it goes with the PC.
If you tried to install it on one of YOUR PCs Microsoft would tell you it is a conterfeit copy.
If you do not know how to install XP then say so and I will give you a quick overview.