If the system is running 64 bit W7, then the issue of a firewall is a little tough. Many have not yet released a W7 compatible version. For 32 bit W7, you can use AntiVir (also works well in 64 bit) and OnlineArmor. The paid version of AntiVir * includes a firewall which does work well in 64 bit, but the free version does not.
I have been using W7 on all three of my systems for a year now, and I haven't had any issues at all. Personally, I think it is the best OS MS has released in many years.
* Edit: AntiVir Security Suite