Well, in my experience installing, that thing usually happens when you put an old OS installation into a new hardware. (Like putting a dual core processor in an installation where it's only a single core previously) So try it.
Question one in the bios does it show the correct CPU being installed?
Question 2 does CPU-Z show it as an E8400?
Go to Run by hitting the start ket and r or just open up run from accesories open up msconfig. Under boot tab click Advanced Options you will see number of processors up top check the box and put it to 2.hit ok apply and reboot.