Microsoft pushed out Build 10565 to Windows Insiders on the Fast ring three days ago. This preview includes Skype integration, Microsoft Edge improvements, Cortana enhancements, some new icons, and support for nested virtualization. It also introduces a change to the way the activation process works -- you can now activate Windows 10 using a Windows 7, Windows 8 or Windows 8.1 product key.
New Fast ring preview builds can usually only be obtained through Windows Update, but Microsoft has today taken the unusual step of making Build 10565 available to download as an ISO to allow Windows Insiders to try out the new activation paths.
If you aren’t currently running Windows 10, and want to try out the latest preview build, you can download the required ISO from here.
If you have a spare Windows 7, 8 or 8.1 product key, you can install Windows 10 on any PC, or in a virtualized environment (such as VirtualBox), and use the key to activate the installation.
The ISOs come in x64 and x86 versions and in a wide selection of languages.