Do you get a POST screen? Like when you start the computer, do you see the various information popping up or a splash screen? This would indicate the processor is activated and working. Its still remotely possible you need a BIOS update.
I just get a black screen, no error message. Is there any way to do a clean install without starting all over?
This is not possible. You can back up your items in an attempt to do so, but installed programs would need to be re-installed. Documents and media files could be moved to a separate drive or storage device. When we use the term clean install, it really means completely wiping out Windows, formatting the entire file system, and starting from scratch. Obviously this is not the best answer to your problem, but it may be the most basic solution. You have replaced the entire "brain" of your computer with a new processor. There is no way for the operating system to understand what has taken place.
One theory we discussed last night was that you could install the dual-core, use System Configuration, and set the system to boot using only 1 processor temporarily, then install the quad core again. However, this is kind of a stretch and very unlikely to work. Its either the BIOS and the motherboard drivers, which kemical will very likely look into for you, or the unfortunate situation of Windows not being compatible for this type of hardware upgrade.
When Windows is installed, the hardware abstraction layer is created to manage, at a very high level, instructions moving back and forth from hardware. What we have debated now is whether or not the new CPU is creating an OS-related problem or if it is a hardware issue. Some people have been able to upgrade their CPU's just fine while maintaining the same copy of Windows on their system. Why it is not working in your case is still something of a mystery, but even if it were not and the computer booted fine, you might find yourself wanting to try to do a clean install anyway (just for housekeeping in this instance and ensuring maximum compatibility).
Reference: Hardware Abstraction Layer. We know unique system configuration parameters are created during Windows installation. These are mostly motherboard related drivers, instruction parameters, and so forth. What we do not know, specifically, is to what extent the Windows Hardware Abstraction Layer may interact with your CPU. If it is reliant on a CPU with an old instruction set and architecture, and a new one is introduced, this could be causing your black screen of death.
Alternatively, if you are getting a NO POST, which is what you indicate to simply be a black screen, and discernible fact that Windows is not trying to boot at all, and you cannot get into the BIOS at all, this would indicate you may need an updated BIOS.