My workstation has been serviced by HP warranty engineer and new motherboard has been fitted along with new PSU.
The reason of this warranty repair was random shut downs or restarts.
After it has been serviced I can no longer activate Windows.
It seems that replacement motherboard has no SLIC table bundled with BIOS. I have checked it with SLICtoolkit.
It means that OEM Windows key can't be injected when activating Windows.
This machine was originally shipped with Windows 8 Pro and activation key was stored in original motherboard BIOS in SLIC table. Since the replacement motherboard has no OEM Windows key I'm not able to activate OS that was part of the purchase.
I have sent ticket to HP support center but got no feedback.