My old Radeon 9200 256Mb did that to me once, I powerd on the pc and on boot, colours and orange and black lines went straight across the screen. I found out that the card wasn't pushed into its slot properly. When i pushed it in properly, my problems were gone.
Tried re-seating many times, card is also held in place by screws and clips, so probably not that. After first installation when it ran for 30 mins OK, it now shows screen corruption immediately on startup.