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How do you get XP Home to recognize and respect each user's choice of default programs?
Assuming that both users actually log out and log in (rather than user fast switching) you could use a .reg file for each user to change the system-wide default at log-in. It's a dirty hack, but without using additional software, that's probably the only way to do it.
Personally, I would ask the IE user why they don't like Firefox and then try to change his/her mind.
Kaled.