Shutdown hangs after having played the game Soldat full screen (Vista 64 bit, Dell XPS 430, BIOS A01)

Hello,

- I use a Dell XPS 430 with a Radeon 3870 X2 graphics card.
- I use Vista Home Premium 64-bit.  
- Some time ago I updated the BIOS to version A01.
- I also went from Vista 32-bit to Vista-64 bit a while ago. I think that was before the BIOS-update. But I don't know exactly when the problems began.
- I also installed Vista's Service Pack 2 a while ago. Plus I have all updates after that.

Problem: When I've played Soldat in full screen the shutdown process hangs: after "Shutting down..." or "Afsluiten" (Dutch) the monitor looses signal, and the computer is having a bit of hard drive activity, but it keeps hanging and doesn't power off. Soldat shuts down properly, no processes left. 

When I don't play Soldat in full screen, the shutdown doesn't hang, and everything's fine... So when Soldat changes the resolution, to be it 640x480 or 1024x768, the shutdown process hangs (when I shutdown) (and I have to power off by keeping the power buttton pressed a few seconds).

The log book doesn't give any errors. Everything seems to go fine. Also, when I start up the computer again, there seems to be no problem.