For installing different versions of Windows 7, you need to have its separate discs for each of the version. The only difference between these disc is the presence of a configuration file called as the ei.cfg. This 51 byte configuration file, ei.cfg, tells the installer what version disc it is. In short, this file decides which version to install, depending on your serial.
While one can always manually edit this file, and manually create an all-in one universal Windows 7 DVD, it’s not so easy.
The ei.cfg Removal Utility is a simple tool that will remove the ei.cfg from any Windows 7 ISO disc image, thereby converting the image into a “universal disc” that will prompt the user to select any edition during setup.
This tool works by toggling the deletion bit in the UDF file table, eliminating the need for unpacking and rebuilding the ISO, which means that this is extremely fast (the process of patching the ISO to remove ei.cfg takes only a fraction of a second), and the process is easily reversible (running the utility on a disc image patched by this utility will restore the disc image to its original state).
On the same page, you will also see The Windows 7 ISO Image Edition Switcher. It is a set of small binary patches (and a tool to apply these patches) that will convert an official Windows 7 ISO disc image into an official Windows 7 ISO disc image of another edition.
The resulting ISO images are bit-for-bit identical with those posted on MSDN or TechNet, and their SHA-1 hashes should match the official hashes posted by Microsoft.