I can now hereby confirm that the problems that I encounter are related to the graphics hardware, NOT Windows XP.
I tried to play it with a Nvidia Geforce 8400GS on Windows XP and this one worked flawlessly.
So I would suggest to incorporate my patch to the release .zip so that XP users can also play the game. Replacing the function call with the older version (InitializeCriticalSection) doesn't do any harm, as no special parameters are used anyway in the InitializeCriticalSectionEx function call.