How can one shut down a computer from the windows 8.1 setup?

Solution 1:

Well, then, again. :)

When at the start of the OOBE wizard, whether after installing Windows yourself or when first booting a OEM PC (I guess), you can press CtrlShiftF3 to enter Audit mode. This mode can be used to customize a Windows installation, much like an OEM does. Although they only do it once, of course.

After getting logged in, there are options for rebooting and whatnot. Just shut down or reboot from there. Rebooting (and then turning the machine off) is probably safer with Windows 8+, considering Fast Boot and all.

There’s further information and an exhaustive guide on SysPrep available here.

Solution 2:

When at the Personalize screen, press ShiftF10 to bring up a command prompt. Once the command prompt is open type shutdown /s. The shutdown command will appear to finish immediately, but if you wait ~30 seconds (add /t 0 , for no wait) (nothing appears to happen during this time) the computer will do a clean shutdown. On next startup, it will take you back to the same Personalize screen.