These articles document that some WIN7 and WIN8 users are seeing early free upgrade offers, based on forthcoming July 29th WIN10 release date

http://www.winbeta.org/news/microsoft-begins-advertising-windows-10-windows-87-users-can-now-reserve-free-upgrade

http://www.computerworld.com/article/2906810/microsoft-seeds-windows-7-81-pcs-with-windows-10-upgrade-nag-notices.html

A user on reddit has today posted a screenshot of what appears to be the first adverts for Windows 7 and Windows 8 users promoting the upcoming free upgrade to Windows 10. The adverts appear in notification form via the taskbar, and allows users to ‘reserve’ their free upgrade to Windows 10, meaning once Windows 10 is available it’ll start downloading immediately via Windows Update.

The advertisement looks to be appearing for all users who installed the optional update KB3035583, which also allows Windows 10 to actually install when it’s available via WU. So if you want the advertisement to go away, that’s the update you need to uninstall. Millions of existing Windows 7 and Windows 8/8.1 users will start seeing these notifications over the coming weeks, hopefully convincing users to upgrade to Windows 10.