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remove your W7 drive
install an old, small hd
install XP
reinstall your W7 drive and chose which drive to boot from from BIOS
Likely the best answer, given the application

XP and 7 could be installed on interchangeable HDD sleds that pop into a
case in a CD bay. Plug in the drive with the OS you want to boot. I put
older Acronis backups on them and use them as restore points when testing
downloaded software. They are unlikely to work properly on a different PC,
even of the same model.

I've set up my laptop to multiboot four ways, from the C: drive, a USB
stick, a CD/DVD and the F12 key (by hacking the Utility partition).

jsw