I've had a Phenom II X2 555 BE for around 2 - 3 years. When I got it, I immediately core unlocked it with my ECS A880GM-A3 mobo, which makes it turn into a Phenom II X4 B55. A few days ago, I installed Windows 7 64 bit to compensate for my 4 gigs of ram. When I start my system with its cores unlocked, it will restart after the BIOS screen. If I disable the core unlock, it boots to OS just fine. My question is: Does 64 bit OS makes a difference in core unlocking? Does my 3rd and 4th core burnt out? Also extra info: I tried core unlocking but keeping the 3rd and 4th core disabled and it still won't boot into OS. Could it be motherboard problems? Sorry for bad English. I will try to give additional information if needed. Thanks! Also it is worth mentioning I'm no computer expert but I tried to make my explanation as short as possible. I also asked my question on TomsHardware, but I had no answer till now so I decided to ask here too. anyone...?