Most off-the-shelf routers for home use will double as, or can be configured as, a firewall.
Note that such router firewalls only block incoming connections, not outgoing. If you get a virus, or spyware on your system, it will still be able to get out. A personal firewall on your PC, such as Zone Alarm, can block this type of communication too.
You can always go for the "belt and suspenders" method, and have an external router firewall, in addition to Zone Alarm, for maximum protection.
The router will provide NAT. What this does is assign each computer behind the router an IP address that is invisible to the outside world. Only your public IP which is provided by your ISP will be visible to the outside world, in effect, hiding your computers. This is a DECENT form of security, but by no means will it make your computer totally secure Won't allow outbound trojans to be blocked). I would recommend a software firewall such as zonealarm to lock it down almost completely (nothing is totally secure. www.zonealarm.com is where you need to go to get this. The free one is perfectly fine (altho I use the pro version, but I am a network admin by trade so I understand a lot of the features in the pro version. You will need to read their help/ user files to totally understand how it works. Can be tricky if you don't know what you are doing. If you go with a linksys router, I can send you a copy of a program that will show you all the in/outbound connections your router makes. Once I installed the router, my zonealarm did not pick up NEARLY the amount of intrusions it did without it, but it still does get the ones that the router did not prevent. Email me at [email protected] if you need any assistance.