OP, try shutting down all of your extra superfluous processes. For me, if I have too many things running in the background, the Source engine won't start properly. Also, for me, it has a REAL dislike for Skype, so close that down as well. Just to be safe, if you're running the Windows sidebar, or something like Rocket/Star-dock, close down that as well.
And stop complaining about your Antivirus, I doubt its that.
You may also want to check that you have the latest drivers for your Graphics card (or god Forbid, that your computer can even run the Source engine... >_> ).