I am not sure if I fully understand your question, are you trying to run Office XP, without using Outlook XP? Outlook XP should not require Outlook Express.
You can set default applications in Vista through the Default Programs applet.
For example:
Start > Control Panel > Default Programs* > Set Default Programs > Set Program Access and Computer Defaults > Custom > Choose a default e-mail program > select the desired email client
* Note: you may need to switch to Classic View to see this option
Hope that helps...