Thanks guys. It looks like I don't have a choice, actually.
I DBANned the drive before starting. XP lived on this drive without problems in my now dead PC when I first built it 6 years ago; can't remember how. My drives may not have been in AHCI mode at that time. But now during installation, it dies with a blue screen:
STOP 0X0000007B (0XF78D2524, 0XC0000034, 0X00000000, 0X00000000)
Apparently this is connected to the fact that it's a SATA drive and XP doesn't have drivers for it. Asus's drivers for it are for installing from within Windows (chipset drivers to be installed first, etc.).
I neither have the patience nor the energy to experiment with slipstreaming.
So I'm going to go with single booting Win7-64 Ultimate and take my chances.
Thanks for link, Chewy. If I can't get XP to run in a VM with the scanner, I'll look into it. There also
Vuescan, but that's money.
Is it possible to create an XP VM with VMWare Player? I know early on there were hacks, I just don't know if VMW closed those or have made it easy now to create VMs through player. There's also XP mode in Win7 that I can try.
Thanks all for the help!