Microsoft has now started sending out notification to Windows 7 and Windows 8 users to reserve their free Windows 10 upgrade. Microsoft delivered this notification through a program which is installed through Windows update. The upgrade assistant is named GWX.exe in C:\Windows\System32\GWX.
If you are not interested in Windows 10, you can remove this program from your PC. Go to Windows Update->Installed update, remove KB3035583. Also, you need to hide the update again in windows updates to prevent it from auto installing again since this is marked “important”.