Installing Windows is not that easy and all it takes is one driver to crash on shutdown and it will restart.
I can't tell if you set Windows to restart on crash or if you checked the Event Viewer. Sorry, but your issue is well worn with no specific cause or cure. But folk by the boatload have seen it.
Ever since I built this PC, when I click shutdown by the start menu it shuts down for a second then reboots. LAN setting is disabled in BIOS to disable wake up from it, and the power button is set to shutdown. I also disabled all other devices from waking up computer. Also when I try putting my computer in sleep mode, it will also start right back up to the resuming windows mode, as it normally does when starting a computer up from sleep mode. Any ideas please tell me. Thank you all in advance.