Windows Vista problem with Switch User

I am running Windows Vista and I still have the problem that when you try to switch between user accounts the computer just goes to a black screen, and the only way to get control of the computer back is to do a hard reboot by switching the computer off then on again. I have searched forums and found other users with the same problem but no solution, I thought that one of the updates would have taken care of it by now. The Microsoft help forum acknowledges the problem and says that is caused by third party software. It offers no solution ... or hope.