blank screen after startup

when i load vista beta 2 i am greeted with a blank screen after its booted. other people have reported having the build number in the lower corner and a moving mouse but mine is totally blank. i can get the screen to work if i boot in low resolution mode (via F8 at startup), but when i try to change over to a higher resoultion or to 256 colour instead of 8bit i get a blank screen. im running a packard bell c3300 laptop (standard fittings except for increased RAM. any ideas?