Mouse spinning in-game. Help!

[Hamsandwich]

When i start Amnesia: the dark descent demo.(Steam) it loads up and I go to the main menu just fine. But when the actual gameplay starts, any interaction with my mouse causes Daniel to go into a spin, staring at the ceiling. It still did this AFTER I bought a new mouse. Other games play fine. How can I fix this problem. I don't think its a graphical problem, It LOOKS fine and i can navigate the level by strafing around blindly. I can lean and open the journals and inventory just fine. Help me!