Originally Posted by qubit
Hey, no sweat.
You'll fix this, I promise you.
EDIT: Reading your OP, I really do wonder if this is being interfered with by your internet security software - which one do you have? It's also possible that you have a malware infection, but I tend to doubt it.
Try disabling your internet security and see if it works. If that doesn't help, uninstall the damn thing and make sure that you reboot. If it then works, reinstall your IS and see how it goes.
Finally, you can have a Half-life 2 for free and completely legitimately.
See this thread: 2 free Half Life games for ati users
I have Microsoft Security Essentials, speaking of which that makes some sense. I believe I had bitdefender last time I was on steam. I will uninstall and get back to you