Jeez.  Its been 3 weeks since I bought the game and up until now, everytime
I tried to run the single player mode, it kept telling me "This game is
unavailable.  Please try again later".  So earlier, I finally got fed up and
decided to uninstall everything, and then after the computer rebooted, I
went into the "Program Files\Steam" installation directory and deleted
everything that was left over.  I reinstalled it, used the original username
and password for Steam that I had before, let it unlock the files and update
Half Life 2, and this time when I clicked Play Game, after about 45 seconds,
it loaded.

YAY!  Anyway, I knew alot of people were having the same problem, so I
thought I would post a message in here letting you know what I did to get it
working.  BTW, on my 1.2 GHz Athlon system, it seems to have a decent
framerate, although it does stutter like most people have mentioned, and I
am only running a Radeon 9600 Pro videocard with that CPU.

Good luck
Sharky