In Windows XP, you can only define one default email client for a computer via the GUI. However, you can give each user own default client by making registry changes. Here’s the edits to switch between Outlook and Outlook Express:

For Outlook:
Location: HKEY_CURRENT_USER\SOFTWARE\CLASSES\mailto\DefaultIcon
Value: "C:\\PROGRAM FILES\\MICROSOFT OFFICE\\Office\\OUTLOOK.EXE\",7

Location: HKEY_CURRENT_USER\HKEY_CURRENT_USER\SOFTWARE\CLASSES\mailto\shell\open\command
Value: “C:\\PROGRAM FILES\\MICROSOFT OFFICE\\Office\\OUTLOOK.EXE\" -c IPM.Note /m \"%1\"

For Outlook Express:
Location: HKEY_CURRENT_USER\SOFTWARE\CLASSES\mailto\DefaultIcon
Value (HEX): 25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,00,65,\
00,73,00,25,00,5c,00,4f,00,75,00,74,00,6c,00,6f,00,6f,00,6b,00,20,00,45,00,\
78,00,70,00,72,00,65,00,73,00,73,00,5c,00,6d,00,73,00,69,00,6d,00,6e,00,2e,\
00,65,00,78,00,65,00,2c,00,2d,00,32,00,00,00

Location: HKEY_CURRENT_USER\SOFTWARE\CLASSES\mailto\shell\open\command
Value (HEX): 22,00,25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,\
00,65,00,73,00,25,00,5c,00,4f,00,75,00,74,00,6c,00,6f,00,6f,00,6b,00,20,00,\
45,00,78,00,70,00,72,00,65,00,73,00,73,00,5c,00,6d,00,73,00,69,00,6d,00,6e,\
00,2e,00,65,00,78,00,65,00,22,00,20,00,2f,00,6d,00,61,00,69,00,6c,00,75,00,\
72,00,6c,00,3a,00,25,00,31,00,00,00

To set other email clients as a user’s default, you can input the required value for the client in the two mentioned keys. To get this value, you could use the GUI to apply the client as the default, reference the key values, and then change each user’s keys accordingly.