In fact, I purchased a version because of this issue. There was something about it if you search either the help on steam or the forums on planet half life.
I can't remember the exact chapter, but if you tell me, I can run a quick game and save it after that point if you want. Is it the beginning of the chase, or the second part after you get the gun?
Even if I do, your shits going to be fucked again next time. Check the forums, you have to delete a file or run something in the console, or.. something. I'm much to lazy to look it up again myself.