First question: is it a property of the COMPUTER or of the USER ACCOUNT?
In the last case, it works as intended for other users on that PC. In the first case it doesn't work for any user. That's an easy test, isn't it?
If it's for one user only, for IE check the accessibility options. For IE6 you can see the "ignore colors" checkbox on http://www.seoconsultants.com/windows/ie/accessibility/
If it's for every user on that machine, I suspect the video card or the video driver. First reimage the machine and if that doesn't help replace the video card and reinstall the drivers.
We have about 100 computers in our network. There is one of those computers that does not display background images/colors on some of our web applications, but they work on the rest of the computers. I am pretty sure the HTML/PHP/CSS code is working (i am using just a bgcolor='#some color') but just in that single computer it is not showing any backgrounds, neither on FF nor in IE. Any ideas?