Screen gets black and freezes when switching users (new installati

Hi,

I recently installed WinXP on two Core-Duo machines. When I switch from one
user to another (fast switching), the screen gets black and the computers
freeze (I need to make a reset to regain control). This does NOT happen if I
log off the user before loging in with another user. I have installed *many*
machines before and never got that problem. Where should I look ? Is this
related to the processor (Core-Duo), the video card, or something else ?

Any suggestion would be appreciated.