1. Hit "run" in the start menu
2. Type "regedit"
3. Open "HKEY_CURRENT_USER\Software\Microsoft"
4. Delete the folder called MSN Messenger
5. After this MSN Messenger Live will login as normal.
or...
If you're not comfortable editing the registry, you can run this small reg. file posted by
Jonathan Kay [MVP] which does the same thing as posted above:
Remove Messenger Settings