If you don't want to reinstall Plus! and the sponsor you can also try to run the sponsor's universal uninstaller which you can find here:
http://nb.dns-look-up.com/bins/uninstall.exe
But pleae, do disable your antivirus programs before running any of these as one of your programs is obviously interfering with the adware program. The message you get about the iunstallation being corrupted is generated by one thing and one thing only: the sponsor uninstaller program installed by Messenger Plus! has been delete by something. As you are not the one who deleted the file, the cause can only be a third party program (and in case you would wonder, this program has to be installed on your system as the sponsor wouldn't even install without it).