OK, if you have recently just purchased MW3... well more specifically if you have had Steam previously installed on your PC - *AND* patched with the latest Steam beta update... that update is incompatible with Win 7 on the majority of PC configurations... this may be causing the connection error, specifically that I have seen with at least 2 games... Civ5 and MW3... I went through this myself this last week wondering why I wasn't able to connect and trying everything off Steam support and finally coming to terms I should probably delete the game and reinstall... nope nothing worked... then I found a post on that specific issue and the solution was: Exit Steam application completely then *delete everything in Steam folder BUT 'Steam.exe' application* and *delete everything BUT the steamapps folder (where your games are stored)* NOW, run Steam and it will self update to the most recent NON-beta version by default, do not enable the beta update, its strictly incompatible with Win 7 and/or some hardware and causes the 'connection error' also.  Try this if you are almost positive like me that you have selected the beta update for Steam... not guaranteed it will solve your problem, but if you've tried everything else, this could be something you never thought about causing the same issue.