I'm running an HD4850 1GB on Windows XP Pro SP3 with ATi CCC 9.4
Whenever I try to set my second monitor to "Extend Main onto monitor", my computer locks up and gives a lined green screen like in the picture below. The one on the left is usually grey, but it appeared black for the picture.
After about 20-30 seconds my desktop reappears, but the computer is still locked up. After rebooting, it works for a few minutes, then freezes again.