In Steam, go to Community tab, click your name to view your profile... halfway down on the right side check your 'recently played game stats' window and see if there are 'ban(s) on record'... possibly you have been caught using hacks? If it does click the ban(s) on record and verify it is a MW3 ban... if not you may be using Steam's latest beta update which has an incompatibility in Win 7 and MW3... to relieve that, exit Steam completely, then in your Steam folder, delete everything but 'Steam.exe' application AND delete everything folders and all EXCEPT the 'steamapps' folder where your games are stored... now run the 'Steam.exe' and it will default to the latest stable non-beta update, then relogin to your account, and you should be able to play... assuming there is nothing else wrong with Steam servers in your area or there isn't a more complicated issue with your network/ISP, etc. causing the errors.