A new Microsoft website has just made life a little easier for anyone who's ever purchased a retail copy of Windows 7.

If your original installation DVD has been damaged or lost, you can now download the installation media in ISO format from the Microsoft Software Recovery site. (Previously, it was possible to download Windows 7 ISO media from Microsoft's online software distribution partner, Digital River, but those links stopped working last year.)

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You'll need a valid retail product key to unlock the download. After entering that key and selecting one of 19 available languages, you click a button that verifies the key you entered. If you pass that verification step, you can download the media. The ISO file can be used to burn a DVD, create a bootable USB flash drive, or install Windows in a virtual machine.

But before you get too excited, consider the limitations. The ISO download is for full retail copies of Windows 7 only, purchased in a shrink-wrapped box or from an authorized download site (including Microsoft itself). It won't work with Windows 7 OEM System Builder media, which is still available for sale from online merchants.

And not all retail keys will work. In my testing, the verification step failed and I was unable to download installation media when I used a product key from a Windows 7 upgrade edition purchased at retail. I was also unsuccessful at convincing the site to authorize a download using valid Retail keys obtained from Microsoft subscription services such as TechNet or MSDN.

If you're hoping that you can use this option to refresh or repair Windows on a PC that came with Windows 7 preinstalled, you're out of luck. Entering the product key from the Windows 7 Certificate of Authenticity attached to a PC in my office produced this error.

If you need to restore an OEM copy of Windows 7, you'll need to find your original installation media, beg the OEM for a replacement, or locate the compatible media from somewhere else.

Curiously, a separate Microsoft-hosted site allows you to download ISO files for Windows 8.1 without entering a product key at all.

Why are the rules for Windows 8.1 more relaxed?

The obvious difference is that anyone can install Windows 7 without entering a product key, allowing all of the operating system's features to be used for up to 30 days. And using a single command, you can "rearm" that unactivated copy for up to three additional 30-day periods. By contrast, Windows 8.1 requires a valid product key for installation.

Hat tip to Lifehacker for first pointing this site out.

Update: For several hours after this story was first published, the Microsoft Software Recovery website was offline. It is now online again.