One interesting thing I've discovered recently is that Microsoft uses a different method to validate Windows if you're using an OEM machine.
Royal OEMs (like Dell, HP, Lenovo, etc.) are only required to put a sticker with the serial number on the outside of the case, but they are NOT required to enter the serial number into Windows.
It turns out that the validation in those cases is limited to checking whether the digital certificate installed inside the BIOS chip in the motherboard matches the one installed in Windows or not. The serial number is just a generic one, and it's always the same serial for all of the machines.
What this means, for us, is that you can buy a machine with the cheapest Windows 7 edition, format the hard disk, and afterwards you can grab yourself a DVD of Windows 7 Ultimate to install it.
Then you just have to install the digital certificate issued to your manufacturer (they are very easy to find on the forums) and .. voila! your Windows 7 Ultimate edition will pass the authentication and validation procedures with flying colors.
It's a loophole that many people are taking advantage of, but Microsoft cannot do anything right now to stop it.
The reason seems to be that OEM manufacturers pressured Microsoft very hard because they didn't want to have to enter a unique serial number for each Windows edition. They just want a generic serial number so it's easier for them to dump the same OS image to all of their hard drives untouched.
I know it's very tempting to try the trick, but remember that it's not legal to do that! Although I really doubt that anyone will notice if you do it 😉