HP p2 1394 Powers up, no boot, no screen, shuts off, repeats infinitely

[Pseudonyms]

Well this might be a simply answered question which will make me feel stupid but somewhat relieved, or it will be a complex question that vindicates my baseless assumption about this machine.

So I got this HP p2-1394 from someone who had no power cord or connections for the monitor, just desktop, monitor, keyboard, and mouse.   for 35 bucks.   I figured even if it doesn't work, I came out on top with a working mouse/keyboard, HP 20 inch monitor, and a case to build my next PC for a friend.  I get home and prepare to plug in your standard AC power cord 120-240v and realize that this motherboard supports DC input, puny little thing I'd never seen such a power input for a desktop.  Now I'm far from a computer guru, but I noticed that this looked like a laptop charger.  I googled the PC specs and found the power supply specs on HPs website.  I looked online and saw that for 30 bucks roughly on amazon I could get a power supply for this machine.  no pickup near by.  Not wanting to wait or trust a non OEM external power supply from a retailer I've never heard of through amazon, I bought a universal laptop charger from walmart.  Matched the specs EXACTLY to the specs of my machines power supply.  get home, find the "C" tip which  is the proper measurements and fits like a glove.

hit the power button

we have lights, we have sounds.  The power button lights up continuously, no blinks.  The orange light turns on a fraction of a second and stays lit until about 3 seconds later when power cuts.  Silence for 5 or so seconds, and the whole cycle starts over on a continuous loop until I unplug the PC.  All the while the green light in the Optical drive blinks and I am actually able to open and close the drive if I'm fast enough, but always the same result.

I have already tested the hard drive in an external dock, wiped it clean from the previous owner, and it is in good health with a copy of windows 10.  That's all I care about, unfortunately, now windows is linked via Microsoft account and motherboard.  So the only way I know to activate this copy of windows is to boot it with THIS motherboard.

have i made a critical error in assuming this uses the same power supply as a laptop?   it matches specs to the T. 

would it work if I bought the recommended power supply for the desktop (which is exactly the same by all appearances and specs) as the laptop power supply?

Is my motherboard dead?

If my motherboard IS dead (no problem I can replace it and upgrade a bit to a real power supply.)  How do I save this legally purchased copy of windows if I never had a chance to link it to my Microsoft account before switching motherboards?

[WAWood]

@Pseudonyms 

If the legally purchased copy of Windows is Retail, then when you reinstall it on another PC and go to activate it, it will probably complain as that copy has already been used (maybe).  All you then do is call MS and tell them you are MOVING the copy from the old PC to the new PC and they will give you a NEW product code that you will enter manually, and then, the new PC will activate Windows without issues.

As to the motherboard being dead, your description certainly seems to indicate that, and unfortunately, while HP does provide hardware diagnostics, it does not provide diags for checking motherboards or their components.

If you want to try running the diags, press the Esc key repeatedly while rebooting and when the HP Startup Menu appears on the screen, select the option to run system checks, usually F2.

[L3ORIC]

Activating liscenced windows in another PC is not a problem. You can contact Microsoft Support with proof of purchase and they can easily set it up for u on another PC.

For motherboard I would ask you to check the Volts of the DC input and buy a cheap DC to AC converter from a nearby electric store to power it up. That is the most easy way to check if the motherboard is working or not.