Funny, I took a class where we did nothing, but play with open source firewalls. Since it's a small amount of pc's and you don't know linux really well I would look at Untangle. Nice GUI interface, easy to setup and maintain, and the community forums are responsive. Showed my old boss and he was considering using it as a back up to the sonicwall we had (not that we had issues, but you never know!).
Open Source Network Gateway | Untangle