Messenger Plus! has an optional sponsor program (which is a customized version of Lop).
(sponsor agreement screen)
If you installed it by accident, you can follow
these simple steps to remove it (also posted by Carltos Cool).
Keep in mind that Microsoft Anti-Spyware alerts the user even when they didn't install the sponsor. If this is the case, you can opt to ignore Messenger Plus!, so that it doesn't affect its functionality.