Windows 7 has a firewall built in. The last few versions of Windows have. Obviously you need to turn it on, and make sure you've got all your security updates installed, but beyond that it's sufficient for basic protection. If you want to go stronger there's myriad options, we use Symantec (not the Norton home crap, but the Symantec business-line products) at my office. Also you could consider a hardware-based firewall, such as basic protection offered by routers (esp. routers upgraded with DD-WRT or other custom firmware), also SonicWall routers offer very good protection and are widely used in the business world. You could bump that up to a Juniper or a Cisco router, or even a dedicated PC on your network operating as a gateway firewall running custom software for even stronger firewall capabilities.
In short, there are too many options to list. Is there something in particular you're trying to protect against, or are you just asking for a firewall "because someone told you you needed one"?
