Found it: ATI's Catalyst driver 7.5 (7-5_vista32_dd_ccc_enu_46757.exe)
caused the problem. I wish I didn't delete 7.4. I've tried to remove,
re-install and remove the 7.5 driver a couple of times and the problem
comes back when the driver is installed. A bug report is on its way to
ATI...

Cheers,
Michael