While playing CoDMW2 online, i got the message that my NAT was strict (so my ports were being blocked). Being familiar with NAT-problems, i searched for the right ports and forwarded them in my router. To my surprise, this did not solve the problem, so i went to check what's wrong. Turns out the UDP ports i opened were still being blocked (checked with PFPortChecker), TCP worked fine though. I initially blamed my router, but when i tried playing CoD with zonealarm off, my NAT was changed to open. So somehow zonealarm is blocking my UDP ports. I ofcourse have the CoD executable on full access.

lastly, some specs which might be relevant:

OS: WinXP Pro SP2 (windows firewall disabled, by the way)
ZoneAlarm version: (Free)

I hope somebody on this forum has a clue what my problem is, because i certainly do not