Microsoft is actively developing their operating system (Windows) as well as other products. Microsoft is currently developing Windows 8 (the upcoming operating system), as per some unofficial source final version of Windows 8 will be released on October 2012. After releasing many previews of Windows 8 including developer preview (build 8102) and consumer preview (build 8250), Microsoft released Windows 8 Release preview (build 8400) two days ago (Thursday, May 31, 2012). Windows 8 Release preview (build 8400) is the last preview of Windows 8, I think after that Microsoft will release Windows 8 beta and after the beta Microsoft will release final version of Windows 8, I'm eagerly waiting for final version.
I suggest you to install Windows 8 along with another operating system (dual boot) or install in virtual machine because Windows 8 preview is under development and contains many bugs.
How to install:
Burn Windows 8 release preview iso to any dvd disk by using any dvd burner (You can use Windows Disc Image Burner which is preinstalled in windows 7) or use Microsoft Windows USB/DVD download tool And boot from dvd and follow other instructions.
Burn a CD or DVD from an ISO file
More info and serial key of Windows 8 Release Preview
Download: Microsoft Windows 8 Release Preview 64-bit (x64)
Download: Microsoft Windows 8 Release Preview 32-bit (x86)