I managed to get mine working. I had posted this on Reddit, and somebody responded by saying they had fixed it by completely uninstalling and reinstalling Steam under a new path. I gave it a go, and lo and behold it worked. Sucks to have to reinstall everything, but at least everything's working again.
I mentioned in another reply, but it's getting really [i]really[/i] infuriating shouting into the void. I just want to be able to talk to somebody.