Microsoft has started advertising Windows 10 to users of Windows 7 and 8 with the window that you see above. First noted by a user on Reddit, this window is now being reported by multiple users that Microsoft is allowing them to reserve a copy of the OS for free.
Thank you to aworrior for noticing this and reported it in this thread:
Edit: Another article, however I can't seem to access it:
Edit 2: Installed the KB3035583 which pushes the upgrade (thank you John). After a restart and a few minutes, the icon appeared in my icon tray.
(French screenshot I know)
Edited by Aura., 01 June 2015 - 05:24 AM.