Make sure you understand the difference between an anti-virus program and a firewall.
They are NOT the same thing and you need BOTH.
A lot of companies who are famous for one thing (like a firewall) are now starting to sell other things (like an anti-virus) so the lines are blurring.
For example Norton, with products like Internet Security Suite (or whatever it is called) include an anti-virus AND a firewall, and a couple of other tools (spam filter?).
Zone Alarm, most famous for a firewall, are also starting to sell other tools including an anti-virus.
AVG, most famous for anti-virus, are starting to sell other things.
So I would go for:
The "free" Firewall from zone alarm.
The "free" anti-virus from AVG.
You may also want to consider Adaware, a free spyware package.
Winpatrol, a free tool to monitor your Windows system to make sure no one is changing anything behind your back.