Here are some utilities, all of which will display your product keys:
Belarc Advisor: http://www.belarc.com/free_download.html
I recommend this one too.
(It does a good job of providing a wealth of information.)
Note: Also check the side of your PC case or bottom of the laptop for the
OEM key code. If there is a sticker with the code it will most likely not
match what is displayed by the utilities mentioned above as OEMs such as
Dell use an internal key code when they build your PC.
The key code on the sticker is for use with your Windows CD
(that hopefully came with your PC)
if you ever need to reinstall Windows.