Every so often my PC has rebooted on its own.

Today Thur 14th Sept. it shut down(logged off ) then Auto started again right in the middle of typing a message on a utility website. Its not the first time its happened. its a pain had to start all over again on the utility website.
Any ideas on why it does it?

Windows 7.
Don't know anything else my son deals with the technical side and its jargon.
Many thanks

Bob, September 2017

reinstall windows

tj, February 2019

You wouldn't drive a car without learning - its the same with a PC.

these may help:

http://www.tomshardware.co.uk/forum/25617-63-computer-restarts-randomly-windows

https://www.pcworld.com/article/2974060/windows/when-your-computer-reboots-itself-over-and-over-again.html

fafsav, September 2017