If you cannot even start Messenger without getting the “Stopped working” message box try to reset the settings in regedit:
Delete the value of the keys:
This should make messenger reset the values and start up normally.
Microsoft suggests to do this as well:
Delete key UTL type REG_SZ with your email in: HKEY_CURRENT_USER\Software\Microsoft\MSNMessenger\PerPassportSettings\<Hash Number>
After sign in remember to change your display image.