Hp pavilion a822n motherboard removal

I am trying to replace the motherboard in an HP Pavilion a822n for a charity. It's a K8S-LA ASUS motherboard. Seven of the screws are standard Phillips head screws; the eighth appears to be some sort of rivet. It has a narrow x in the top, is smaller than the other screws, and will not turn. Looking from the back, there's a solid metal inset corresponding to this "screw" rather than a standard rivet. From the side, this "screw" connects to the case.

Any suggestions for removing this last "screw"? The process can be destructive, as the old motherboard is totally fried, and the seven standard screws should suffice to hold the new motherboard in place.

Thanks.