Specifically, by the demo I mean near fully functional OS that you can install anywhere to try out. This ability means you can download the Windows 10 ISO right from Microsoft and install it on a home-built PC, or any PC for that matter. Even Mac users can do this and install the OS using Bootcamp .
How it works
Anyone can go and download the Windows 10 ISO from Microsoft. Simply navigate to http://www.microsoft.com/software-download/windows10.
If you are on a PC, you can download the Windows 10 Media Creation tool that lets you make an ISO for use on a DVD or USB key. You can then use this to install Windows 10 on any computer. The process allows you to pick the language, edition, and architecture.
You do not have to Activate Windows 10
Perhaps one of the little unknown secrets about a Windows 10 installation is that you do not need to use a product key during the process.
Oh, Windows 10 prompts you for it, twice in fact. However, in both cases you can hit the little 'Skip for now' button and let Windows 10 install. It is tiny, but it is there.
So what happens? Nothing. Windows 10 Home or Pro is installed, and you can use it. There are just two limitations that I have come across:
Watermark on the lower right-hand corner saying Activate Windows
You cannot use anything under Personalization e.g. change wallpaper, accent colors, lock screen, themes, etc. All options are there, they are just grayed out and inaccessible
Other than this you have a fully-functional Windows 10 OS. There are no nag prompts either and yes, you get all system updates (at least for the year).
You can enroll in the Windows Insider program. However, this just removes the watermark, but it will not allow you to personalize the computer.
The video of how to activate windows 10 professional: