MS Windows
Everyone here is somewhat right on the licensing terms for OEM software, but not entirely. That's because MS has different licensing terms for OEM software, as well as different types of OEM software.
My company is registered as a system builder with MS. I can buy OEM Windows XP from a distributor and install it onto my customer's pc when I sell it to them. I must install the software using the OEM preload method, and the customer must have the Out Of Box Experience when they turn on the pc for the first time. If they do not buy an entire pc, then there are qualifying products, which are a hard disk or a mainboard.
Should this pc catch fire, the customer may transfer this license to another unit, as long as the entire package is transferred, ie the CD, the booklet and the Certificate of Authenticity (COA)
This is an open OEM license, and generally costs about US$130 in this country.
There is a closed OEM license also. These are the OEM licences that are sold to vendors like Dell, HP, etc. In the licensing terms, the customer is not allowed to transfer the COA to another pc except to hardware from the same vendor. In other words, if my Dell pc caught fire, I could buy another Dell pc without windows, and install the previous software on it.
Although the terms of the closed OEM license says NO to installing to other hardware, MS so far has not rejected, say a Dell COA to a HP pc transfer.
There is also a Volume Licence OEM. These Windows XPs are popularly known as "Corporate editions", where one license key can be used for multiple installs. This is usually the type of software that is counterfeited - a counterfeit CD, COA and booklet is sold to you. It will activate, but it will not validate for windows updates.
Example
Support for all the above is from the vendor.
There are Windows upgrade packs. The license is only valid if you upgrade from a qualifying software product, and you must have that product for the license to be valid. So, if you get Windows XP upgrade from Windows 98, then you must have both Win 98 with you (but not installed anywhere), as well as Win XP upgrade COA. You can transfer the license, as long as terms for both the products are kept. There are no OEM upgrade packs.
Finally, there is the retail version of Windows. Comes in a box, manual, etc. Expensive, but you can buy it without a qualifying product, and you may transfer the license any time and to any hardware. Support is from MS.
An open OEM key is interchangable with a closed OEM key and vice versa, but they are not interchangable with a VL OEM, nor with a Retail key.
All basic versions of windows are the same. How they behave and the license key they accept is determined by the contents of "setupp.ini" in the i386 folder of the install CD. Of course a vendor may install other software and customise the OOBE.
Hope this helps.