A separate router is undoubtedly best but it's not the cheapest. If you have Windows 98 or above, all you need is a network adapter for each PC (which may be built in already) and a cable. To connect two PC's you need a crossover cable. If you want to add more you'll need a hub OR you can use ethernet - but only if your network adapters have coax connectors on them. These are getting rare.
That's the hardware sorted out. The software is already on your Windows disk. It's called Internet Connection Sharing. Alternatively, there used to be a program called Lansuite which was free for home use. Have a look around the internet.