Brian Freeman found a flaw in my articleReinstall Windows when you've lost your reinstall disc or partition. I told readers that "when it comes time to activate Windows, use the activation number on your PC." But Microsoft no longer requires vendors to make this number readily available.

Every legal copy of Windows has its own activation key, also called a product key or a product identification (PID). It's made up of five groups of five characters each, and it's unique to your copy of Windows. Basically, it's a proof of purchase. You can download and install Windows for free, but without a unique activation key issued by Microsoft and not in use on another computer, it won't work for long.

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When you install Windows, you'll be asked to type in this key. If you bought Windows by itself, the key is in the packaging. If your PC came pre-installed with Windows 7 or an earlier version, the key is printed on a metal plate on the back or bottom of the computer.

But that's not the case if  your PC came with Windows 8 or 8.1. Microsoft no longer requires manufacturers to put this key on your computer, and the manufacturers as a rule don't do it.

So what can you do if you need to reinstall Windows 8? You could send your PC back to the manufacturer and pay them to reinstall. Or you could buy another copy of Windows--effectively paying twice for one product.

Or you can take some precautions beforehand.

If your PC is still running, you can get the activation key from Windows itself. The apply-titled program ProduKey does this job quickly and easily. The program is free and portable; you don't have to install it. You do need to download the right 32- or 64-bit version.

You can also get this key from Belarc Adviser, a free (but not portable) program that offers all sorts of useful information.

Once you've got the key, put it somewhere safe. Mr. Freeman suggests taping it to the outside of the PC.

One more suggestion: When your PC is set up the way you want it, create an image backup on an external hard drive. This provides you with a quicker and easier way to restore Windows. You won't need the activation key, and the restored version will have your programs and settings.

Windows 7 and 8 both come with image backup programs, but I prefer the more versatile EaseUS ToDo Backup Free.