On a machine running XP Home and used by two people each with their own (admin-level) login, if one user changes the default browser, it changes for the other too. However, one user wants Firefox, the other wants IE7.
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.