Seems to be related to reseting power management settings to default, which was throwing an error up and not allowing you to do so. I believe that has been sorted now but lots of people are finding that when they boot their PC from cold, the Windows 8 blue logo appears and the little icon whirs around indicating the system is starting then just before the desktop should appear the screen goes black. Monitor is still on as is PC, fans spinning etc but the desktop either never comes up or takes between 5 and 20 mins to appear. A lot of folk have uninstalled Avast to find that this action completely cures the system of the issue.
However......I've just uninstalled Avast and I'm still having cold boot issues as above. Next step for me is to boot to safe mode and try fixmbr from a cmd prompt which appears to be a solution of the Microsoft website.