I have a funny feeling he is wondering about wether or not buy the full Retail copy or the OEM copy.
Buy OEM if you plan on keeping your computer mainly the motherboard for as long as you are happy with Windows 7 if your looking at upgrading your motherboard in the near future buy the Retail copy, this allows you to install on as many different computers, the OEM copy is locked to the motherboard.
I have recently bought the OEM copy of Windows Home Premium 64Bit, coming Win Xp 64Bit and I dont plan on upgrading for another couple years on the 775 socket, does everything I need it to do and more just now. Plus I saved about £70 going OEM