how to purchase a windows 7 product key

You can't purchase just a key, it must be purchased as either a part of a retail upgrade or full version package; or a OEM System Builder License Package.

Because Windows 7 is in limited stock because Microsoft ended sales in October 2013, this has created demand because of its scarcity. This means retail boxed copies demand a premium, while OEM System Builder copies remain affordable.

Full version (Retail):

- Includes transfer rights to another computer.

- Doesn't require a previous qualifying version of Windows.

- Expensive

Upgrade version (Retail):

- Includes transfer rights to another computer.

- require a previous qualifying version of Windows.

- Expensive, but cheaper than full version

OEM System Builder version:

OEM versions of Windows 7 are identical to Full License Retail versions except for the following:

- OEM versions do not offer any free Microsoft direct support from Microsoft support personnel

- OEM licenses are tied to the very first computer you install and activate it on

- OEM versions allow all hardware upgrades except for an upgrade to a different model motherboard

- OEM versions cannot be used to directly upgrade from an older Windows operating system

There is nothing wrong with using it too, especially since all future releases of Windows will be full versions anyway.

You can find OEM System Builder software from dozens of online merchants. The current price for OEM Windows 7 Professional at Newegg, for example, is $140. When I checked a few minutes ago, Amazon was offering OEM Windows 7 Professional packages from multiple sellersat prices ranging from $101 to $150. When I checked just now, a package specifically intended for refurbished PCs cost only $50 for a 64-bit copy.

There are no technical limitations to prevent you from using OEM software on your own PC, although this software will work only for a clean installation and not for an upgrade. In the past, Microsoft has been remarkably inconsistent in its advice to customers about whether this practice is allowed. (See "Is it OK to use OEM Windows on your own PC? Don't ask Microsoft.")