DirectX is updated.
Sound driver completely updated
Disabled onboard sound card.
Did the clean boot thing.
Tried the lowest and hightest resolution and details settings (and variations).
But no joy.
(Now I remember why I bought a console!
If I keep flipping in and out of resolutions then every now and again it'll flip into something which looks correct, but as soon as you do something else (change menus or start a game, it returns to it's previous state.)
I also tried running the second episode and it's the same deal.
Anyways. I'm going to assume at the moment that my current PC isn't supported. I'll just have to wait until I can afford a new PC before I get to play. Anyways, thank you very much for the help.