About a month ago, after updating Windows or something, my dual boot start up shows Win 10 with a border around it. I doesn't change anything, I'm just curious as to how it happened to show up.
I'm guessing that it's just to show the default selection. If you press the down arrow key it will probably move to Windows 7, and you can probably press Enter to select whichever choice has the box.
Behind every good computer... is a jumble of wires 'n stuff.