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Generally I would say just stick with the Windows 7 firewall, and if you are having trouble, try uninstalling your current anti-virus program to see if it is causing a conflict. Other than that there are a number of free firewalls available. I like Comodo, but like a lot of things, you have to be careful in what you select on initial install, or you could get unwanted junk like toolbars thrown in. I think ZoneAlarm is ok too.