I am sorry to hear you are having this problem. When this sort of error occurs, it can either be a result of files on the system, or networking.
I recommend that you;
Go to C:\Program Files(x86)\Steam\SteamApps\Common\Call of Duty: Modern Warfare 3\Players2, and delete your mp.cfg file.
Once this is done,
Launch the Steam application, and select Library.
Right click on Call of Duty: Modern Warfare 3 Multiplayer, and select Properties > Local Files tab > Verify the Integrity of the Game Cache.
This will generate a new config file and will immediately tell you if this is a file problem.
If you continue to have problems beyond this, then the issue will be a networking problem. If that is the case, please provide us the model number of your Linksys router so we can help you get this solved.
First thanks a lot for the quick answer.
I tried out your first step. I deleted that file and after that i did what u said. But the check of my files was negative, there is everything all right with my system.
So i thing its a problem with my networking system.
I have got a linksys router wireless-G-ADSL-Home-Gateway.
The model number is: WAG454G
Hope so much that u can help me :/
No worries! Here are the steps to try and get this resolved.
First you'll want to make sure you have a static IP for your PC (http://portforward.com/networking/static-win7.htm)
Once you have a static IP for your PC, you'll want to forward the ports for the game to be able to communicate properly with our servers.