Lucy,

In Vista go to Start > Control Panel (make sure you are in Classic View) > Double-Click View32-bit Control Panel Items > Double-Click Mail > Click Add > Enter a profile name > Click OK > Then set up your profile according to your settings for POP/IMAP/Exchange ( I like to check the box "Manually configure server settings or additional server types" because the wizard doesn't always works so well.

Be sure that when you are done, you go back to the Mail Icon > Click Show Profiles > Select 'Prompt for a profile to be used' so that when you open Outlook it will ask you which account you want to log into.

Hope this works for you!