tart1 - if you're using MS Outlook or Outlook Express, then you can set identities up on the software. Click on "File" and you will see "manage Identities". Switch on password protection and set yourself a password.
Most importantly, while exiting Outlook, dont click on the Cross on the top right corner but go to File and "Exit and Log off identity".
If your colleague has an email ID of her own, set up a New identity and ask the resident IT boffin to fill out the settings on to that identity.