Screen Freeze forcing reset

Hi
I upgraded to windows vista and the first thing that it complained about was
my ATI graphics drivers - which I updated. I have a motherboard with an ATI
Xpress200 controller and a saphire x1950 graphics card. I installed ATIs
control center for my card and windows seemed happy. However, consistantly my
screen will freeze allowing no input of any kind. My machine is still up and
running - though maybe in standby - but I can't get any response. I have to
reset the machine each time. My machine has 2GB of ram and my performance
rating (for running vista) is 4.3.
Any ideas would be much appreciated.

This seems to be the exact same problem reported about RC1 by Marc last year.
I have tried reformatting my drive, installing XP (with no additional
software/drivers), then upgrading to Vista. Once upgraded I installed the
graphics drivers and windows updates. There's no usb devices attached, no
printers installed, no bluetooth. But the system still freezes periodically -
doesn't usually last more than 15-20 minutes.

Ian