Wrong Aspect Ratio in XP on Vostro A860

I recently put Windows XP SP2 on my Dell Vostro A860. Everything runs fine except that the highest resolution available to me is 1024x768 as opposed to the optimal 1366x768 which is the widescreen (16:9) version. I've installed the latest drivers, but nothing changed. I get a fine resolution using both Vista and Ubuntu, but XP can't seem to accept it. Is there anything I'm missing? Any advise?