This is from Steam Help:
http://support.steampowered.com/cgi...2V0IEVSUk9SOiBXU0FFQUNDRVM*&p_li=&p_topview=1
This is from Microsoft Developer Website:
http://msdn2.microsoft.com/en-us/library/ms740668.aspx
While I was looking, a lot of people referenced that it could be firewall related.
If it was me, just as a test, turn off your firewall and try to connect, if it works, well you need to adjust the settings on your firewall or you need a different firewall. For me personally, i use ZoneAlarm, whil I can play while it is on, i get better connection speed with it off. My other system uses pc-cillan, I have to shut that one off completely.
If it is not the firewall, the steam help forums may be your best bet.
good luck