Hello and welcome to Tom's Hardware Forums.
Firstly, it's best to edit out your e-mail address using the Quick Edit button, before it gets sold to the Spamming idiots.
At boottime, your system seeks out any way of booting up and the one your message describes is a network boot from another machine. The way to stop that message is to tap whatever the first screen you see says you need to go into BIOS or Setup. Somewhere in there will be a setting to remove that from the list of boot options.
That said, so long as you have the system up and running with no other problems than this one, it's as well just to leave it there as a minor annoyance because one day, it may come in handy to boot from elsewhere in your network.