Windows 10 Transformation Pack can make any Windows version look just like the new Windows 10 that will debut on July 29, so if you want to get an early look at this new operating system, this is the best way do to it without having to install more than just a simple app.
Version 4.0 brings all the goodies that are available in Windows 10 preview build 10130, obviously except some features, but as far as the looks are concerned, it gets pretty close (you won't get a dark theme and some new UI features though).
The change log reveals that Windows 10 Transformation Pack 4.0 comes with system icons, sound scheme, cursors, and wallpapers from Windows 10 build 10130, as well as with some changes that could make old Windows look brand new.
For example, developers updated Windows XP's file copying dialog to match Windows 10's layout, while also tweaking transparency to look just like the one that's going to be integrated in Microsoft's new operating system.
Overall, Windows 10 Transformation Pack is a good opportunity to make your desktop look fresh, but keep in mind that before installing anything, you should create a restore point to make sure nothing bad happens if the OS look is messed up.
Once you do that, you can safely download Windows 10 Transformation Pack and get a taste of Windows 10 on your Windows 7 or 8.1 PCs in just a few minutes.