Users can get free trial of any Windows 7 version

A Windows 7 installation disk can be tweaked to install any version of the operating system, according to a popular newsletter revealed.

By deleting one small text file from a Windows installation DVD, users can choose to install any of five different editions, giving users a "try-before-buy" opportunity before upgrading to a more expensive edition, said Woody Leonard, a contributing editor to the Windows Secrets newsletter.

Leonard published step-by-step instructions that walked users through the process on Windows 7 RTM, or release to manufacturing, the final build of the operating system that Microsoft has already shipped to computer makers and distributed to IT professionals and developers.

The procedure hinges on deleting the "EI.cfg" file on the installation media, said Leonard. According to Microsoft documentation, "EI.cfg" is a Windows Setup-specific configuration file used to determine what edition and licence will be used during installation. Earlier versions of Windows used a file called "PID.txt" for the same purpose.

"If you have a physical Windows 7 installation DVD ... [you can] use either gBurner or ISO Recorder to rip the DVD into an .iso file," said Leonard, "then follow the instructions above to delete the EI.cfg file and burn a new DVD."

Leonard recommended a pair of CD/DVD tools, including gBurner System's gBurner and ISO Recorder 3.1, for transforming the installation DVD into an .iso file. Once they have an .iso in hand, users can then delete the "EI.cfg" file and then burn the .iso file to a new, blank DVD for installation.

Although the process is elaborate, and probably only for the technically astute, Windows Secrets editor Brian Livingston said it was really the only way for users to try different versions of Windows 7 before they plunk down their money.

"I think this would be of great interest to corporate IT administrators," said Livingston. "They will be able to put [Windows 7] Professional on one machine, and Home Premium on another to test each out before deciding which to buy for what group of employees."

The procedure also offers a way to try out a more expensive edition of Windows 7 before paying for an Anytime Upgrade, the in-place updates that let users bump up to a higher version. Microsoft sells an Anytime Upgrade from Windows 7 Home Premium to Professional for $90(£54), but doesn't provide any trial or grace period; users must pony up the money to obtain the key that unlocks the Professional-only bits within Windows 7.

Leonard's try-before-buying stratagem isn't original; other users posted instructions on how to delete the "EI.cfg" file to bring up a Windows 7 installation edition choice screen within weeks of the launch of the OS's public beta last spring.

He was, however, one of the first to confirm that the tactic still works on the final build of Windows 7 that will go on sale 22 October.