The only problem is the Windows Firewall only blocks or patrols incoming traffic and it can be easily turned off by another application, possibly a worm. Another draw back to Windows Firewall is that rival firewall makers claim that the API used to manage the Windows Firewall could also be used by attackers to modify the software or turn it off.
The fact that it does not monitor outgoing traffic and that it can be turned off easily by other applications really doesn’t make it a safe firewall in my opinion.
