Your computer should not reboot itself when you aren't there, unless you've asked it to. Few things annoy me more about Windows than when I return to my desktop only to discover a Windows Login box. Almost as annoying as this is that Windows Update pesters you to reboot your computer after applying updates, even while you're working. Nevermind that it steals focus from other windows, interefering with whatever it is you're getting paid to do, the damn thing doesn't have a "NEVER" option to choose from, just "now" and "later"!