Okay, thanks for the info.
Hmm, buying it through the main steam store collection is about as basic as you can get in terms of what should work. And the exact same thing works for your friend, but not you? And his was the gift copy, while yours was the regular "purchase for myself?"
I just went through everything and I'm puzzled that I can't see anything wrong in the settings. I'll probably have to get my Valve technical contact to help out, but he's quite quick at things. I'm puzzled that the gift copy would be the one that works while the direct purchase would not, though; if anything, I'd expect the other way around.
Can you try a couple of things?
1. Run a verify integrity of steam cache on the game.
2. Tell me what version number you are seeing in the upper left hand corner when you start the game?
3. PM me a list of all the files and folders in your root install folder for the game.
Sorry for all the trouble!