|Install Windows Server on one of the PCs.|
Configure Remote Installation Service on the server.
Configure an XP image for RIS (you will need an XP CD for this).
Do a network boot of the PC to install the OS (you will need a valid, unique XP key to complete this process).
You may find it easier to just use a CD to install directly. If you don't have a CD drive for the computer it's going to be a lot less trouble to just shell out a few bucks for one rather than bothering with remote installation.