Well, technically, you can change the ram, CPU, hard drives and what not. There is no 'special' driver that drives the RAM, PSU, or even the CPU. The Motherboard, that is a different beast altogether.
The motherboard will be loaded down with specific chipset that the previous OS has not loaded. In the past, I have tried transplanting harddrives from one system to another, and usually it has failed due to a drastic change in chipset architecture on the motherboard that the only other option is to reinstall the OS to be functional.
You can transplant HDs from one system to another system, provided that the OS was installed for the particular board, otherwise, the OS will start up freaking out about how none of the operations it is trying to do is giving back what it is expecting. Short of actually creating a very 'universal' type image, which sometimes is difficult in and of itself.
I believe the current model for Windows OS is that if it detects enough system changes, however, it may want you to re-activate your windows install to avoid 'duplication' of the license, and that would come from multiple changes to make it really question whether or not this is the computer it was installed on or you cloning your drive and sticking it into another similar system.