What has happened is she setup her MSN Messenger account on your computer using the Windows Messenger that is intergraded into XP. Windows Messenger is also tightly intergraded with Outlook Express so this is why you see her contacts when you start OE. Here is what you need to do to stop her MSN Messenger account from starting with OE.
First, make sure that OE and Windows Messenger are not open. Then open the control panel and select "User Accounts". Next, click on the "Advanced" tab in the User Accounts windows. Then click on "Manage Passwords" under the "Passwords and .NET Passports" section. You will most likely see an entry that says something like "Passport.NET\*(Passport)". Just click on remove and then click OK. The next time you open OE, Windows Messenger should not sign into her account anymore.
Hope this helps.