There is an XP driver for it at the link below:
Have you tried it?
Hope this helps.
Hi, I just got vista and my Acer 640U scanner driver won't load. I've tried VueScan, which has been recommended over at Microsoft, but when I open it, it says there's no scanner connected. Vista tells me it can't install the driver, that there's not enough room (?!) Anyhow, I know I may have to buy a new scanner, but if anybody has a workaround, I sure would appreciate some help. --Katie