Musicmatch must be set as the default player for all media types??? (from your post it pops up while doing things on the PC). 

 

There are options within MMJB for file types.  You can uncheck the types you do not MMJB to be the default media player.  Also uncheck the box to "reclaim file types" or every time MMJB starts it will make itself the default again.

Sysinternals have a great, free, tool to identify all programs that startup automatically during boot.

It's called autoruns.

 

MusicMatch may have put an entry in the HKLM\software\microsoft\windows\currentversion\run.

That entry can be safely deleted.