Check which SP level of XP you have loaded. (Start, Control Panel, System). This will tell you if you have Service Pack 1, 2 or 3 installed. It could be that, if you have an early SP and haven't updated to the latest SP level, then Windows Update won't be able to work. There are plenty of places from which you can download SP2 and SP3 (as necessary).
Best Black Friday Deals 2017