2. I tried enter maintenace mode
Every time I put old CPU it was ok to boot Windows 8.1 x64.
But latest BIOS is 1 year later after E8400 was issued, so it shoudl support it.
I think this is kind of bug in BIOS that it writes something CPU depended during flash.
reflash with E6750 ,didn't help. Only re-flash with E8400 inserted helped to resolve my issue.
Before re-flash BIOS does not see Intel Virtualization option with E8400 inserted - after re-flash it shows it.