Windows: Windows 7 install discs are edition-specific—if you've got a valid license key, you still need the right disc for installation. The ei.cfg Removal Utility, however, lets you create a new, universal installation ISO for Windows 7.
Read more at lifehacker.com
Although your Windows installation disc may say "Home Premium Edition," it still contains the other versions (such as Pro or Ultimate) on the disc—it just has a very small file called ei.cfg that tells the disc what version to install. The ei.cfg Removal Utility creates a new ISO of your install disc that ignores this file, thus letting you choose what edition you want when you start the installer.
While this tool is especially useful for computer technicians who have to install Windows a lot, it's also nice to have for those of us that tend to lose discs often—especially in this day and age of emailed product keys. Now, if you need to reinstall Windows but can't find your disc, you can just borrow a friend's—even if it's an edition that's different than yours. You just need let the installer know which edition matches your product key (Note: Your disc still needs to be the same kernel as your key (i.e. 32 or 64-bit).
The ei.cfg Removal Utility is a free download, Windows only, and requires a Windows 7 install disc to use. Thanks, Cody Ryan Harrod!