If it has anything to do with proprietary hardware then it has nothing to do with the card you bought. It means the motherboard is made to work only with the GPU that came in your computer which is why this one won't boot up. You're more than welcome to return your card and try their theory but I'm betting you'll find that you're stuck in the pre-built proprietary pandora box.
770s lacking backwards compatibility is BS lol. Any aftermarket card will fit in any 16x PCI-E slot regardless of the board. In this case it's your HP board that is the issue.
Replacing a motherboard is as easy as anything else in a computer. All you need to do is take the GPU out, unplug the cables from the board, lift it out, remove the heatsink, properly clean the heatsink and processor with a Qtip (or coffee filter) and rubbing alcohol, place CPU in new board, replace heatsink with new thermal paste (grain of rice size in center of CPU, don't spread), put board in case, plug back in. The only issue you might run into is Windows carry over.
Newegg.com - ASUS M5A78L-M/USB3 AM3+ AMD 760G + SB710 HDMI USB 3.0 uATX AMD Motherboard
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