My current setup is a laptop (XP Pro Sp2) on a wifi network. The wifi is wpa-psk secured, with a few other security measures. I even tangled with the hardwall firewall and added a few rules. Now, like a few others have said, unless I don't trust the insiders using my computer...a software firewall shouldn't be neccessary. Well, I do have a softwall firewall (Outpost Pro). Why? Because I think the Hidden Process Monitor and Component Control are excellent features. I'm also a firm believer in layered security. Is it overkill? probably.
I would never knowingly do harm to my computer but, occasionally, things do inadvertantly happen and I want to be prepared.
[Back on Topic]
In the absence of a router or any other firewall, Windows firewall is better than nothing as I said in my first reply.