I take it you have an older video card? Might be a good idea to upgrade. A real basic Radeon 9600 is fairly inexpensive and can still deliver nice quality. I've had a similar problem to yours before, but it was because I was on 32bit color depth. I just waited for the next release which solved the problem
What is your video card anyway? I'm guessing some generic built-in chipset since, to my knowledge, nVidia and ATI only offer 16bit and 32bit color depth. Though there's not a major difference between 24bit and 32bit...I think