I haven't actually done this so I don't know all the details but Windows comes with a simple built-in peer-to-peer network capability. So, with the proper hardware which may include two network cards and cables, and possibly a router, you should be in business. Check this out in Windows help.
I have also used a a product called AnyPoint (< $200 for two PCs). It comes with everything you need. It does require that the PCs have modems installed and both modems must be connected to the same phone line so if you have only one jack, you'll need a splitter. This product uses the already existing phone lines in your office to network the PCs. They don't need to be in the same room, just connected to the same phone line. It also allows the networked PCs to share the same internet connection and printer which is why I was using it at home.