When un-installing Plus! and the sponsor, did the sponsor uninstall validation window pop up, which is asking you to type a code displayed on the screen to verify you are human? And did you reboot after the uninstallation?
What can help in some situations:
- Disable any anti adware software you might have running
- Install Messenger Plus again, with the sponsor
- Uninstall Plus! and the sponsor again
- Reboot
- Enable your anti adware software again, if you have any.
Installing Plus! and the sponsor again might help fix a damaged uninstaller.
Also see
How to uninstall ad-ware/sponsor.
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