I had a similiar problem and tried the solution below, and it worked for me.
HOWEVER: You will be deleting a steam file, so I take no responsibility if it doesn't work for you. And i suggest backing up the file before you do so. However this seems to be a common problem with the new update..
hopefully it will fix it for you...
In this directory
C:\Program Files (x86)\steam\SteamApps
(of if you have a 32b os)
C:\Program Files\steam\SteamApps
there is a file called:
multiplayer ob binaries.gcf
close steam... completely... delete that file. then restart steam, you should be good.