FireFox vs IE - How to keep them from battling each other?

Okay. I use FireFox, its set to my default browser.
My wife only wants to use IE.
We use Win XP, and we each have our own Logins…
But if she sets IE as her default browser, it sets it on my login, and if I set FireFox to my default, it switches it on her side…
How can I fix this?