Though I'm less clear why someone would want to do that. If backwards compatibility is the concern, then XP would be the better choice for a 32 bit backup - whether real or virtualized.
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Win 7 32 offers far better support for hardware and offers many great built in's that simply xp don't have. I was actually thnking of doing the same hence in my searching on possibilities is how I got to this site.
I want to do dual boot simply for hardware testing purposes before I swing over to 64 bit for good. Had an issue with TV Tuner on 64 where 32 worked like a charm.