I found this useful article over on Online Tech Tips

Those of us who install or update our operating systems through downloads rarely remember to write down the product key. Or, if we do remember to write it down, you can almost bet we will misplace the Post-it where we scribbled it.

So, to save us all the heartache of not having that expensive product key in the event of a system crash, let’s all recover our Windows 7 or XP keys right now and save them to an external hard drive, or at least a USB drive. Here’s the easiest way to do it:

Go to WinKeyFinder.com and download the Simple Keyfinder. It is a very small 38k zip file. The product information says it works with Windows 2000, 2003, XP, and Vista, but we tested it in Windows 7 and it worked like a charm.

Double-click the zip file you just downloaded and choose extract all files at the top of the menu.

In the menu that appears, browse to choose a place for Simple Keyfinder to extract, then click extract at the bottom right off the window. Make sure the box is checked next to Show extracted files when complete.

A new window will appear listing the contents of the unzipped file. Double-click SimpleKey_Finder. If windows gives you a warning screen, click run.

This will open a window telling you your product key.

Don’t worry about writing it down—the program creates a text file containing the product key in the same folder where you extracted it. By default, the file is named WinKeyFinder_found_Key.

Save this file to an external hard drive or USB drive so you will have it if you need it. System crashes occur more often than any of us like to admit, but now we will all be prepared to restore our operating systems.