What model is your scanner?
I had the Visioneer 7600, which was unsupported on XP. HOWEVER, I got it to work by using a set of instructions on Visioneer's website:
XP Install for 7600 USB.
Those instructions may work for you (regardless of your model number) or the website might have a similar article for your scanner.
The scanner works great in XP, by the way. I've never had any problems.
- rp