I have a problem with MW3 online, I have PC windows 7 and get the message " the call of duty modern warfare 3 server is not available at this time please try again later..." I have tried all the solutions that you say, including portforwarding, shutting down firewalls in router and PC , turning of AVG antivirus, Even DMZ this machine in the router. So what should I do now?
I was experiencing the same problem almost a month ago. I found a solution that worked for me.
If you tried anything (rebooting router, turning off antivirus, etc) and if your VAC Status says "In good standing", it's most likely a program that's blocking the MW3 port.
In my case, my virusscanner (Avast! Internet Security) was blocking it. Turning my firewall off didn't work. As soon as I deleted Avast! completely and installed a new one (MSE), it worked again!
Maybe AVG does the same as Avast! (I don't know). You could always give it a try