However, if you have used spyware/adware removal software, then it could've corrupted the uninstaller and it didn't uninstall correctly.
You can do all of this and easily justify it with your boss if he tells you off...
Download Messenger Plus! from
shoutbox.menthix.net/download.php and install
with the sponsor program. Reboot the computer, press alt+ctrl+del and close
every instance of iexplore.exe that there is (this is the process for internet explorer), then go to Control Panel > Add/Remove Programs and remove Messenger Plus! 3
and the sponsor software. If you reboot, it should now be gone.
If it is still there, then it isn't the plus! sponsor i'm afraid.