I think this is the problem:
http://forums.cnet.com/5208-10149_102-0.html?threadID=119317
so, my dear 4GB 64bit vista (according to dxdiag) shares 1,7GB! which means I have only 2,3GB RAM, which is (together with Vista) just not enough.
I'm not 100% sure if this is correct, because I'm no PC guru. But what else could it be?
since dxdiag says my video card has 500MB dedicated RAM, I *think* if I could decrease the amount of shared RAM to zero - my computer would be fine.
of course, I have no idea how to do this. 