I get a "Windows cannot find gpedit.msc" error message, presumably because I have the Home version rather than Pro (though I don't really care as to the reason I get the error, to be honest). Which unfortunately for MS does nothing to increase the likelihood that I will buy the Pro version (of Win8 or any future edition), and instead decreases the likelihood that I'll ever buy another MS product at all. If anything this behavior makes more sense in a professional (i.e. IT Department-coordinated group policy settings) rather than a home setting, so for me to have no way to remove/adjust this "feature" really turns me off of buying Windows for home use.
Oh if you have a home version, you won't get the group policy editors. Actually the policy makes the most sense for home users. Send the editorializing to MS, not me.
Open an elevated powershell prompt and run this (it's one line):
New-ItemProperty "HKLM:SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\au" -Name "NoAutoRebootWithLoggedOnUsers" -PropertyType "DWord" -Value 1
Or this, if you don't want to use powershell, open an elevated command prompt and run this (all one line):
reg add "HKLM\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU" /v NoAutoRebootWithLoggedOnUsers /t REG_DWORD /d 1 /f



