Windows 7 Home Premium 64 bit - it's an old HP laptop with a built in Intel HD Graphics chip. DirectX 11.
It's as soon as I enter a new doorway. I tried deleting the folder in Windows Explorer that belonged to the doorway, and the game would then load with me in that deleted folder as a tiny room with nothing really in it. I could then walk back to level 1 (C:) - however as soon as I try and walk into a new doorway, the same shader bug occurs.