found an answer
While the WinXP guess starts to initialize from the CDROM, it puts a message on the bottom of the screen about hitting F6 to load other drivers. At that time if I hit F5, it will eventially ask what kind of computer. If I pick Standard PC, it shuts off some of the ACPI stuff, I think. Anyway, that lets the WinXP install complete the first phase. You have to modify the xen/domainame file as is explained in many other discussion on xen and WinXP to continue with the reboot and complete the install.