Hi, and welcome to the forums
What is the exact model of your motherboard? Check your motherboard manual. Are there any settings in the BIOS or jumper settings on the board itself that you have overlooked? If there is a setting in your BIOS to disable one of the processor cores then try it. On some BIOSes you can select which core to disable for troubleshooting purposes. It is possible, but unlikely, that one of the cores on your processor is faulty. Is the board actually capable of supporting your processor? What readings are you getting on the 'System Health' page in the BIOS? Did you install Vista with all 4GB of RAM installed? If so, try reinstalling with just 2GB and then upgrade to 4GB afterwards.
Dwarf