I've installed the game on two systems (both running Windows Vista Home Premium 64-bit) and had the same problem, at different points in the first mission. On the first computer, I reached the point with the tribal hostages, at which point I accidentally Alt Tabbed out of the window. When I returned, my characters refused to move. Reloading my weapons worked, and so did firing, but whenever ordered to move, any of the three characters simply turned and was stuck in the same spot, no matter what level their action points were at. Ending my turn filled the action points, but my characters still exhibited the same behaviour. Loading previous saves didn't help at all; they refused to move no matter which save I loaded.
On the second computer, I'd only just spoken to Charon and crossed the first bridge when the same problem hit me. I hadn't saved yet at that point, so I exited the game completely, opened it again, and started a new file. My characters moved at first, but froze at the same point. In all cases, I was using a custom character.
I hope there's a fix for this, I was really enjoying what I could play of the game. Thanks!