Windows through the ages:
http://www.engadget.com/gallery/wind...ough-the-ages/
After upgrading my desktop I pulled the product key using the Produkey utility. It is different than the Win 7 product key.
Here's an Engadget review:
http://www.engadget.com/2015/07/28/windows-10-review/
(one thing the reviewer says it that he doesn't know any reason they skipped windows '9' - but there is actually a
very good technical reason which he apparently isn't aware of)