I guess it could be because IE9 is probably built upon new APIs that came only in Vista, where Windows switched to HW accelerated windows management (which was

I guess it could be because IE9 is probably built upon new APIs that came only in Vista, where Windows switched to HW accelerated windows management (which was pretty fundamental system-level change): http://en.wikipedia.org/wiki/Windows_Display_Driver_Model http://en.wikipedia.org/wiki/Windows_Display_Driver_Model