Opengl fullscreen problem

Ok I have nvidia geforce 3 ti 200 with 27.42 drivers, on win xp. I have this strange problem with all opengl games. When turning on a opengl title, like wolfenstein, my screen starts to flicker and looks like two different screens civer each other ( if I may call that this way). However when I press alt + enter a almost full screen window shows up with the game and… It doesn’t flicker anymore !!!
When I turn back to full screen it comes back to previous state.
My question is: What the hell is going on?
Thank you for your help.