There are two versions of Windows 10: the one you buy at retail or obtain from the Get Windows 10 free upgrade system, and the testing version available to members of the Windows Insider program. The latest test build -- version 10565, available to Insiders who are on the fast ring (the group that gets the quickest updates but the most instability) -- introduced a helpful new feature: you can now activate Windows 10 using a product key originally meant for Windows 7, 8, or 8.1.
This streamlines your ability to create a fresh installation, instead of upgrading on top of an older version of Windows. Right now, the officially approved method to get a fresh install is to upgrade, then format and install Windows 10. Assuming that this new feature makes it into the retail version of Windows, you'll be able to skip the upgrade step when installing Windows 10, as long as you have that older key.
Also, right now some users have trouble successfully activating an upgrade to Windows 10. The official solution is to re-install the upgrade, which can be a time-consuming hassle. Activation allows you to do the fresh install after doing the upgrade. This is important if you need to reinstall Windows 10 later but no longer have access to the product key for the version of Windows that was originally on that device.
With the new activation method in build 10565, you can simply use your current Windows 7, 8.0, or 8.1 product key to activate instead, and your copy of Windows 10 is permanently linked to that device. With this setup, you no longer need to enter any keys when you install Windows 10 in the future, because the operating system has already sent a kind of digital fingerprint for this device to Microsoft.
As with previous test builds of Windows 10, version 10565 brings other improvements as well. Microsoft Edge, which replaces Internet Explorer as Windows 10's default Web browser, continues to get tweaks, though it does not yet support Firefox or Chrome add-ons, as Microsoft has promised. However, when you hover your mouse pointer over a tab, a thumbnail preview image now pops up, which can make navigation faster. Edge also now lets you sync your bookmarks and Reading List items across Windows 10 devices.
Skype (purchased by Microsoft in 2011) is getting support built into Windows 10's Messaging and Phone apps. The company has touted being able to respond to an instant message or text message via Action Center notifications, instead of needing to open up a program. This integration is also coming to a test build of Windows 10 Mobile in the near future.
For those of you with styluses or touch screens, Cortana (Windows' version of Siri or Google Now) will be able to take written notes and use them to generate an itinerary for you, and it will also now check your email for things like movie tickets or events. You'll get a reminder two hours ahead of time, and Cortana will integrate with Uber to arrange transportation.
Cosmetic improvements to Windows 10
Windows is getting little design enhancements, such as title bars with a color saturation that better matches the color theme that you've chosen for the rest of the UI. There are also more detailed context menus for the Start menu and new icons for the Device Manager and Registry Editor.
The Windows 10 Insider program is not currently open to new users, as Microsoft wants to encourage people to get the retail version instead. The website says "The next wave of the program is starting," but this statement has been in place for several weeks now with no new info. When that changes, we'll let you know. Microsoft also does not give specific dates for when Insider updates will be available to users of the retail version of Windows 10.