As everyone knows (or should), Windows 7 is published in a number of “editions”, like “Professional” and “Home Premium”. But within each of these categories there are different price points. Caveat emptor, as we used to say.
For example, Windows 7 Premium 32 and 64-bit can be purchased in its “Full Retail” package. It is also available in an “Upgrade” package. Yet another bundling is labelled “OEM”, supposedly designed for “system builders”. Microsoft says explicitly that an OEM license is tied to a particular computer, and can never be moved to another.
The major issue with that last is not that Microsoft requires the purchaser to agree never to bother them with support questions (“free” support questions, as if there is such a thing), but that changing system hardware components — or a reformat — will require a call to Microsoft for re-authorization of the license.
This is actually not new. The same “feature” was gently introduced with Windows XP, and always a lurking presence in the lives of hardware hobbyists (and others) saddled with a PC built by one of the big vendors like Dell or HP. What is new is Microsoft’s policy that changing the motherboard constitutes making a new computer (see Article ID 424125. Officially, at least, they will not re-authorize a copy of Windows 7 on a computer whose motherboard has been swapped out.
This policy also affects the “Upgrade” package in a roundabout way. If you buy a computer from one of those aforementioned big OEMs, you will almost certainly have been delivered a computer with the OEM version of Windows. If you then replace the motherboard you will be unable to reinstall the old version of the O/S that is prerequisite to doing an upgrade.
So the bottom line is… most of us who have old computers are probably better off getting the full retail version. Except that it costs twice as much as the OEM, and around the same order of magnitude more than the Upgrade.
And people ask why I run Linux.