Windows 7 has moved along since version 6.1 Build 6519.1.x86.fre.winmain.071220-1525 (first Windows 7 mliestone). From build 6519.1, Microsoft went up all the way to Version 6.1 Build 6574.1, made available in March. But even with the development milestone label of 6.1.6574.1, Windows 7 is still at the Milestone 1 stage ThinkNext revealed. With this release, Microsoft has tweaked the installation process of the OS only superficially, but now the License Agreement references Windows 7 instead of Windows Vista. The next version of Windows still features the same kernel as Vista, the move to MinWin has yet to take place, and even the same editions, namely the Windows 7 Business, Home Basic, Home Premium and Ultimate. The new Windows 7 installer no longer requires the introduction of a product key until the finalization of the installation process, and after the initial setup, and even at that time, the phase can be skipped entirely, permitting users to run the operating system as a trial. Windows 7 also creates a hidden 500.0 MB partition which is not accessible via Windows Explorer, and is dedicated to management purposes. Windows 7 M1 Build 6.1.6574.1 features a touched-up graphical user interface. Windows Explorer comes with two new buttons, one for Views and the other for switching the preview panel on or off. The March edition of Windows 7 M1 brings to the table the Health Center, complete with mouse over reactions. Windows 7 M1 Build 6.1.6574.1 also permits end users to temporarily disable the User Account Control in scenarios where the UAC requires multiple confirmations.
Info Source: Softpedia
Here is a link to a most-likely fake Windows 7 Milestone 3 screen shot: http://www.liewcf.com/blog/wp-content/uploads/unofficial-windows7-logo.jpg
Windows 7 Milestone 1 R2 features video