Start --> Run
code:
C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\RegAsm.exe /unregister "C:\Program Files\MessengerPlus! 3\Plugins\Music Logger Plus.dll"
Then, delete "Music Logger Plus.dll" in the Plugin directory.
NB: If the paths are different on your computer adjust them accordingly.
I guess alternatively you could open up the registry:
Start --> Run --> "Regedit" [enter]
and do a search for all references to "Music Logger Plus.dll" and erase them manually.