If you choose to download Windows Vista instead of buying the disk, you will find that the process of burning
the operating system to a disk is not quite as streamlined as you would expect. Windows Vista copies purchased and downloaded from Windows Marketplace are not ISO files. But Microsoft is not delivering Windows Vista as an ISO image via Windows Marketplace.
“That is the way that MSDN distributed the Betas and RCs to developers but is not what we are releasing from Windows Marketplace. For Windows Marketplace, we wanted a solution that would not require special hardware (DVD burner) or knowledge (how to use DVD burning software to burn an ISO image),” revealed Rob Mensching, Microsoft Software Design Engineer with the Windows Marketplace Development team.
This means that users that have acquired Vista via Windows Marketplace will not get an ISO file, but three files: VistaSetupPrep.exe (73.76 MB), boot.wim (116.31 MB) and install.wim (2.25 GB). These values are approximate, as they are associated with either the 32-bit or 64-bit editions of the operating system.
“After all of the files are downloaded, you can launch VistaSetupPrep.exe (or simply click the "Install" button in the digital locker assistant). VistaSetupPrep.exe then extracts the 500+ files compressed inside it and links the .wim files into into a folder called "Vista". The end result is that the Vista folder is a replica of the Windows Vista DVD. Before VistaSetupPrep.exe exits, it launches the setup.exe in the Vista folder and Windows Vista setup starts, just like if you had launched it from DVD,” Mensching added.
But there is also a way to transform the three separate files into an ISO image that users will be able to burn to a disk. First off, you have to post all files on C:. Then run VistaSetupPrep.exe in order to extract the files of the operating system. Next, download CDImage, which you will have to track down and download yourselves, for various reasons that I will not get into here.
Now launch “cmd” and enter "cdimage -m -u2 -bC:Vistabootetfsboot.com C:Vista C:Vistax86.iso" to create the Windows Vista ISO image. Next use your usual DVD burner to burn the Vista ISO to a DVD, just make sure that you use the minimum speed.
Source:→ softpediaMicrosoft, Windows Vista, ISO, Image, Burn, CD, DVD, Tips, Tricks