I am pretty sure i remember what i did. The interesting thing about steam and the new Source MODS is that there are many different instances of hl2.exe that all sperately need to be able to access the internet. Sometimes McAfee gets confused. Try this:
Go into McAfee.
Go to your Firewall. (Mine says something like Personal Firewall Plus)
Go to View the Internet Applications List
Click on New Allowed Application
Navigate to: C:/Program Files/Steam/SteamApps/emailAdress/counter-strike source/hl2.exe
Select that hl2 and allow it.
Now try playing.
Let me know if it works. Once again, i did it awhile ago and i'm not sure if my problem was the same as yours but it's worth a shot. I just checked and i actually had to do that for each game (Dod, CS, hl2) so i have 3 seperate instances of hl2.exe being allowed.
Ace
Edited by ace_porate, 18 November 2005 - 11:06 PM.