If ur grandma PC restarts every 15 minutes it is almost certainly because of a Virus .. If I am not wrong it is Lovegate.....

This virus was very active 6 months back.. It used to restart the PC after prompting...

It is fairly simple to remove the virus from the system... Give this a try first... Than if still the problem persists... u could think of her PC being hacked....

Frankly I would see no reasons why anyone would like to break into her PC.. still ask her to check her bandwith consumption with ISP .. bcos Incase her PC is used as a scapegoat for launching attacks..