From the top of this forum (not this thread) find the Search this Forum button (at the top-right) and click on it. Type 640P into the search box and click the button out to the right of the box. You will see dozens of threads identical to yours with solutions on how to make a parallel port Acer scanner work in Windows XP. After all, that's what is a forum is for (or did you think it was only for posting identical requests in?) It takes less time to find those previous threads than it did to create your request.
The answer doesn't just reside in the 640P threads for Acer scanners. The same applies to ALL parallel port Acer/BenQ models. This is one of the very few parallel port scanners that will work in Windows XP period. There is no other brands of parallel port scanners that will work in Windows XP. Windows XP just doesn't like most parallel port scanners.