Change or Switch user causes problem

After i logged in in A and then log out and want to log in in B, i just get a black/dark blue screen, and i can’t do anything at all, instead of log out or restart(Taskamanager works). After that I am not able to log in in any account on my pc.
The nearly same problem is with switching from user A to B. but if i log out from B, i can go back to A.

I have tried it with 2 computers, both with VIsta Home Basic 32bit. On operating with UAC, and the other without.