Enjoy your new toy, but If this starts bothering you again, just stop disconnecting the PC from the mains so it doesn't think there'd been a power failure when you plug it back in. That way, it won't do anything unless/until you press its power button.  I'd connect it to a surge protector as an extra precaution if you decide to leave it plugged into the mains. In this case, don't unplug or turn the surge off. If your question has been answered please mark a post(s) in this thread as the Solution...thanks!
I’ve just bought a Dell desktop Inspiron 3910. Is it normal when I plug the computer into the mains supply that the computer will start up temporarily and within a few seconds turn off , this all happens before I’ve touched the on and off switch on the PC ?   Thank you 
Dell Desktop Inspiron 3910 power issue