Yeah, Discworld Noir is a wellknown bastard when it comes to running it under XP. Had exactly the same problem myself.
Took me quite while to find the solution, but in the end, what Damien mentions did the trick.
Replace the "tin3_dxd.exe"-file that you get when you install the game with the fixed version from the underdogs. That should do it. Unfortunately, this gives you another bug, which is the fact that you can't load your saved games from the game's main menu, so you have to skip through the intro scenes every time you start up the game, and then load it. A bit of a hassle, but worth it, since you get to play a truly great game.