Newer HP Pavilion has the Black Screen of Death with a blinking cursor

A friend asked me to look at her business computer because the HP tech they sent out couldn't figure out the problem with it. Well I started out eliminating all unnecessary stuff on it Like the optical Drive and removed 2 or the 3 sticks of ram it had in it. Also I unplugged the Card reader to. I also unplugged all of the other stuff in the back except the monitor and the keyboard and mouse. well it still did what it was doing. So I then pulled the hard drive out and ran it threw a SMART test and a few other test and it passed. So I then went into the bios and did the default setting and still the same problem. So I reset the bios and replaced the CMOS battery ans still no luck on getting past the problem. So I then plugged up the optical drive back up and ran 1 stick at a time threw Memtest and all the stick passed with no errors. Well the problem was still there so I replaced the SATA cables and tested the PSU with a PSU tester and had no luck at fixing the problem. So with what I have done does this mean the board is out or does it have a corrupt OS on the hard drive. Any and all advice will be helpful. Thanks.