I have the same problem... Now i reinstalled the Steam to C: and download the game again(now the C drive is the default to Steam). The game didn't start, a dll was mising(mss32.dll). I found that on the web, and copied it into the windows folder, then i got an another error
So now... download from Steam in progress... wait and see.
Ok, done.... so.. I had to reinstall the steam to c: then the default steam folder will be c: too. The patch cant find the game so i had to let that to make a custom folder. When the install has finished i just copied the contents of the folder as Jack wrote earlier. In drive c: the game doesn't find the ms32.dll file, i have no idea why. So strongly recommended to install the game to drive C, didn't work from other drives to me.