I understood. Everything from the start again then! Boring but seems the only way

Windows (All versions currently) build themselves around your systemboard/BIOS/CPU/RAM etc to build what is known as the "HAL (Hardware Abstraction Layer) that all your software then feeds from. If you change that HAL dramatically, you will either end up with a blue screen of death, or a system that has so many faults, it makes the blue screen of death look like child's play.

 

Its basically pulling the foundations out from under a sky-scraper.

 

Win7 will also only allow you 3 hardware changes before you need to re-activate it. (Things like Graphics cards, sound cards etc, non-critical equipment)

 

OEM will NOT work on new HAL, as it will simply detect that its a different PC (Different BIOS & Chipset) and you wont be able to activate it, whereas a retail version allows you to install it on a new PC every week if you bought one weekly, just not simultaneously :-)

 

That's why OEM is cheaper, because its a 1-shot deal for the life of that hardware.