I have found a thread with a similar problem...
The short of it says that they were/are having a similar issue at power on. Thier problems started with a bad BIOS update from HP.
I know none of the posts in this thread mention a BIOS flash but the similarities shouldn't be overlooked.
Since the computer will not POST then could our problems be a corrupt /malfunctioning BIOS?
I will wait to see what Scanyman reports back with. They may replace his MoBo without diagnosing the source of the problem.
A new BIOS chip should be cheaper than a new MoBo for those of us who are out of warranty.