HELP! XP Only gives me 16 colors for anti-aliasing

I have an Asus TnT2 and currently, through the new Detonator drivers, I was able to initialize anti- aliasing for my gaming. But I also run Windows XP Pro, and if I leave Anti-Aliasing on, my desktop drops to 4 bit color and I cannot change it thereof. If I set my performance/visual effects of my desktop to “best performance,” my video setting return to normal. Can someone help me? I have tried microsoft’s KDB but found no help there. THX.