XP comes with a built in firewall which is not turned on by default (that will be fixed with SP2). Since Symantec can't hit your system - it would appear that the firewall is doing its job. Along with SP2, changes to the firewall to allow easier user configuration will be added. Another choice is ZoneAlarm - a free firewall. Do a Google search for it, download and install it. Make sure you turn off the XP built in firewall if you're using another firewall. ZoneAlarm is pretty easy to configure, most of it will be done automatically. After installation, hit Symantec's site again and run the security check.