This little freeware utility might be worth a try:
http://windowsxp.mvps.org/defaultbrowser.htm
quote:
DefaultBrowser allows you set the default web browser for your user profile in Windows XP, without affecting the other users. Note that the user interface in Windows XP does not provide an option for setting the default browser for each user. The browsers you see in the Preset Configuration are tested and working flawlessly as user defaults.
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