In Windows XP if you have multiple users set up on one computer then the default browser and default e-mail client settings are computer-wide settings, not user specific. This could lead to a problem if for example you want Firefox and your partner wants IE as the default browser. One solution is to get a new partner, if this seems a bit drastic then these 2 utilities may be of use.

DefaultBrowser 1.4 and DefaultMail 2.1 are freeware utilities that enable you to set per-user defaults in Windows XP. They are written by Ramesh Srinivasan, a Microsoft MVP for Windows Shell/User.

His site also contains many useful tips for Windows XP.

If you prefer to set user-specific settings by editing the registry then you can follow the procedure detailed here - How to Register an Internet Browser or E-mail Client With the Windows XP Start Menu