Difference between Windows install key and serial number?

While poking through the registry the other day, I found that there is a 20 character "productID" under the CurrentVersion key for HKLM\SOFTWARE\Microsoft\Windows. Obviously, this isn't the key used during the installation process, as that key is 25 characters long. I know that older Windows versions (95/98/NT) used to store the key used to install Windows in the registry, but I thought that the newer versions (XP/2000/2003) encrypted the key and stuffed it somewhere more secure (WMI, perhaps).

The "productID" in the registry on XP/2000/2003 is probably some kind of serial number, but can anyone here tell me if I can rely on this number to be as unique as the install key? Anyone know where this "productID" comes from? Is it tied to the install key? I'm working in the Compliance field now, and I have a user who would like to scan his systems for invalid copies of Windows. If I write a script that will scan for this "productID", will I accomplish the same thing as if I had known the actual install key? Will a system installed with the same installation key have 2 different "productID" numbers? Any assistance would be greatly appreciated!

right click on my computer click properties. under registered to: there should be a product ID see if this matches. If thats what you are seeing then read this

http://www.microsoftcalsettlement.com/FindIDNumbers.htm

If I remember correctly on older OS's you could use this Product ID to activate Windows but I dont believe it will allow you to do that in Windows XP. I do know that some parts of the key will tell you if its a valid key or a key that was blocked by microsoft (pirated).

No, this key or Product ID is not the license key use at install time. It is generated by the license key at install time, though. That long, convoluted sequence you enter when manually installing WinXP is used to generate the product ID you see in the registry and in the MyComputer/Properties window.

How they get from the 25-character sequence to the 20-character product ID is something like a reverse hashing process.

Anyway, the product ID can be used to match against the Microsoft data provided by Ghostie, but it won't help you install any new versions of Windows.

Thanks Ghost_25_inf and rapier57. Your replies were very helpful!