the best thing you could try is:
- Download Messenger Plus! from http://www.msgplus.net.
- Install it with the sponsor.
- Reboot your PC.
- Now un-install Messenger Plus!, make sure you follow the instructions exactly there to get rid of the sponsor properly.
- Reboot your PC again.
That should work. The reason that you first need to install with the sponsor is because the sponsor un-install package may have been damaged, read more
here.