Xanex is right, it is the Messenger Plus program that is the root of this. I've only seen 2 ways to get rid of this pest, and niether one was a definitive answer. One was to uninstall the Messenger Plus from add/remove in the Control Panel. If that didn't work, you were supposed to download the Messenger Plus installer again, and install it, making sure that an option for the "sponsor" was checked. Then uninstall it again, making sure that no browser windows are open.
The other way was to download a program called new_uninstall.exe (new version of the old LOP uninstaller) get it here :
http://lop.com/new_uninstall.exe