Its just as good as zonealarm for blocking incomming traffic.. its not quite as smart as zonealarm however.. IE, When a person connects to an IRC server, port 113 needs to be open.. So with XP, you have to have it open all the time, or close it when ever its not in use.. which is kind of a pain for me.. But zonealarm will open it when its needed for legitmate things, like IRC servers, and close it when its not required..
WinXP Personal firewall does not block out-going traffic, so if you were to install something, or something that was installed along with another program without you knowing, that program could send out info from your computer, or your computer could be used in DDoS attacks..
XP Firewall is sufficient, but I would recommend Zonealarm in hopes of cutting down on DDoS attacks
