What you see in Properties is the
Product ID which is 20 characters (you seem to be missing a # up front).
The Product ID is generated when XP is installed from your 25 character
Product Key - that is what you have on the sticker (hopefully). What you see in Properties is not generated until you do the installation. There is some method to the Microsoft madness, but I forget what it all is.
You cannot find your 25 character Product Key in the registry (prevents piracy), but it is on your sticker and you can use third party tools to reveal the 25 character Product Key - like the ever popular:
http://www.magicaljellybean.com/keyfinder/
I have all that jellybean stuff on a piece of paper with all my original installation CDs and paperwork. You never know when you might need it.
You can view/change some other interesting Properties information in the registry here:
HKLM\Software\Microsoft\Windows NT\CurrentVersion
I am not sure what would happen if you were to change the Product ID in the registry though - would you suddenly become invalid?!
People that buy second hand computers are most anxious to change the "RegisteredOwner and RegisteredOrganization" part once they discover their new bargain system is registered to some guy they met last week at the flea market.
They should really do a total reinstall on a second hand system anyway but they never seem to have the installation CD or 25 character Product Key, so we'll fix it so it at least "looks" right and make them feel better

.
If you ever have dealing with Microsoft, I think they want the Product ID.
This post has been edited by joseibarra: 01 November 2009 - 09:28 AM