Every legally purchased copy of Windows has a product key (which, generally speaking, is unique to that copy of Windows), and the product key consists of 25 letters and numbers, in groups of 5, separated by hyphens. In order to install or re-install Windows, you need to enter the product key. Similarly, if you drop off your computer for repairs at a computer store, and they think they might need to reinstall Windows to fix your problem, they might want you to give them your product key before you leave so they can do the re-install.

If you bought Windows from a software supplier, the 25-digit product key should be on a sticker on the CD case (along with a warning saying [ahem] "Don't lose this product key"). Even iff you bought your computer from a store with Windows already installed on it, they should have supplied a CD that contains the software pre-installed on the machine, and somewhere on that CD or in the accompanying materials, should be the Windows product key.

If you can't find a product key from any of those sources, it is not a good idea to ask a friend if you can borrow their Windows product key. Each product key is only legally supposed to be used to activate a single copy of Windows. (It may be technically possible to use a product key to activate Windows more than once -- rumor has it that the limit is about five times -- however, that is permitted for the purposes of letting people re-install Windows on their own machine if they need to. It does not permit the same key to be used to install Windows on multiple machines.)

Instead, there is a program you can download to tell you what the product key is for your currently installed version of Windows. Go to the home page for Magical Jellybean Keyfinder and click the link to download the program. (As of this writing, the download link on that page says "Download Latest Build".) Once you have downloaded the keyfinder zip file (called keyfinder.2.0.1.zip, although future versions might have a different version number in them), double-click the keyfinder.2.0.1.zip file to open it:

(Different computers open zip files differently -- mine uses the WinZip program -- so the zip file might not look exactly like the screen above when you double-click to open it, but the contents, including the "keyfinder.exe" program, should be the same.)

Then in that window, double-click keyfinder.exe, and Keyfinder should run:

I've blanked out the product key that the program displays on my machine, because if I didn't, someone else reading this webpage could use the product key to illegally activate their copy of Windows :)

Note that on my computer, Keyfinder also displays the product key for the copy of Microsoft Office that I have installed.

Bennett Haselton is a technology and political blogger who writes Windows Tips & Tricks by day and writes articles about how to bypass the RM SafetyNet filter by night.