PC won’t power down all the way

This is a brand new PC, I just put it together. While it’s on it works great, but it won’t shut down all the way. It appears to be shutdown, but the Modem and the router shows activity. When I flip the O/I switch on the back of the PC the activity stops but the PC won’t start up. When I flip the O/I switch the other way the activity starts before I even turn the PC on. Power management problem I can’t figure out. Help is needed here. Thanks, Kenny