michaelw, did you try to lower the ram speed in the bios? Some months ago on asusboards, there were many people complaining about not being able to install XP on a new sis648 board, the asus P4S8X. It worked fine with win98. That was a buggy motherboard, with an unstable memory controller (from a bad chipset batch). I also bought one

although I did have some problems, I was able to install XP fine. First and Last asus product I bought
I advise you to go
http://abxzone.com/ forum (formerly asusboards.com) They chat mostly about asus products there. Friendly bunch, I'm sure you'll get help, and I bet many more people are having troubles with the same motherboard you have.
Good luck.