I just have the default Windows firewall, really. Other than that I have the Microsoft Security Essentials, but I have _every single_ automated function on that turned off and only use it for periodic (and manually initiated) scans. I think our router has a built-in firewall, too, but that shouldn't interfere (generally firewalls seem to me to be more in the way than they're useful, and all those antivirus programs that insist on doing things themselves slow down my computer too much, so I don't have much).