Honestly not trying to be a jerk, but you do know that Windows XP does this out of the box? Admittedly not with the flexibility of a "real" firewall program, but I've been using it for years without problems.
Or is this a Linux thing? If so, I thought that was in the OS as well, at least the sharing bit...
My recommendation would be to buy a Linksys or a D-Link router. You get the firewall, the connection sharing, and the reliability of a standalone device that will be a lot more reliable then a PC that is vulnerable to crashes. Right now, you can get a Linksys router with 4 10/100 ports for $50.
I would have to agree with those that suggested the Linksys or D-Link router. Cheap, easy to configure, more options. The new ones have VPN ability built right in. Also you will consume a lot less electricity with a router on 24/7 than another PC.
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