Dell usually deals with this type of problem on laptops in its FlashBIOS updates. Go to the Dell site to see if there's an BIOS update for your model which deals with the problem. Dell usually says that this update has been verified on the following operating systems and lists them out. XP may not be included if your model is fairly old.
Also check what is called "Dell OS updates" for XP if any. I never quite know what it is for, but try it anyway.