Download and install Smoothwall or IPCop works great. They are stripped down versions of Linux, have Snort, IP Tables, etc. and they are free. I'm using Smoothwall right now and have been for years.

cheers

edit: Gotta type faster I guess, and d0pp was using only one hand

You'll need one linux compatable nic in the XP box and two in the older box. The installation instructions will describe their categorization and purpose.