There are different types of licenses for Windows. At the most basic level there are three types: Retail (FPP or Full Packaged Product), OEM (what vendors like Dell and HP use) and Volume License.
Windows OSs are not available to purchase as volume license to begin with. You have to either purchase a machine with Windows preinstalled (OEM) or buy a retail copy. After that you can purchase upgrade OSs under Volume Licensing, this would allow you to have one key that could be used on all machines.
As far as product use rights for Windows OSs -- You can only use it on one machine per license.
Windows applications, such as Office have different product use rights. They usually allow for use on one desktop, one laptop, and one remote connection used by the same user, but these rights can differ depending on product version.
Last edited by kfawcett; Oct 7th, 2009 at 1:00 pm.