Try this mate - It may have something to do with Windows storing usernames/passwords:
Remove/Add Items from remembered list in Messenger login
When you sign in to Messenger, it remembers every email address / .net passport that you signed in under. To remove an item from the list of remembered names:
1. In Start menu select Run
2. enter: "control userpasswords2"
3. Go to Advanced tab
4. Select Manage Passwords Button
5. Add or Remove items at will
Let us know!
Cheers
Lee
