The Lillster

Windows Firewall is decent enough as a free firewall, as it comes with Windows (located in the Security Centre). If you don't wish to use this, then as a guess i'd go for the one that PC Tools offer

http://www.pctools.com/firewall

I'm not all hot on the Firewalls as such, as i use Kaspersky Internet Security, which is an all in 1 Security pack. It's a pay per year one.