I have since reformatted the system and reinstalled Vista, this time without SP2. Same problem.
Also without an LCD monitor connected in the VGA port, but only the TV, the system starts full screen in BIOS etc. The screen only switches to non-full screen mode after Windows starts, so clearly it is a Windows settings and/or a driver problem. Same thing, I can set it with full screen with the Size adjustment, reboot it perhaps once correctly, but is lost soon after.