update-rc.d is a command to set and edit the scripts within the different runtimes. For instance – when you make a script which you need to run each time you start runlevel 2 ( default in debian based systems ), you run the command:

sudo update-rc.d script defaults

I have had some trouble with mpd not being able to startup correctly. I therefor removed the startup scripts in the different runlevels with:

sudo update-rc.d -f mpd remove

I now start mpd manually at startup. More on the subject here: http://www.debian-administration.org/articles/28