Do you mean you are trying to retrieve the serial number from your current installation? If you do not have a licensed copy, give Microsoft a call, as they will be more than happy to provide you a new serial number.
I can only think of one reason why anybody would want to extract the Product Key of an installed copy of Windows. But like what CimmerianX said, provided the benefit of the doubt, you can use any of the programs on the link he provided. I have personally used "Magical Jellybean" before to extract a product key from an installed copy of Windows and it worked perfectly. You can choose any one from the link above.
Note: reiterating what CimmerianX have mentioned, If you have an OEM install, the PK is pretty much useless for reinstallation since all OEM copies are pre-activated and are basically just images and is not the same as retail versions of windows.
Once you've found the Product ID (Control Panel > System > 2nd to last line) and the Product Key, you might find the following useful (I haven't tested it - I'd recommend googling those commands for verification):
Posted on this forum, March 16, 2013
3 posts since Jul 2012
Reset Windows Activation/Remove license key
Open a command prompt as an Administrator.
Enter slmgr /upk and wait for this to complete.
This will uninstall the current product key from
Windows and put it into an unlicensed state.
Enter slmgr /cpky and wait for this to complete. This will remove the product key from the registry if it's still there.
Enter slmgr /rearm and wait for this to complete.
This is to reset the Window activation timers so the new users will be prompted to activate Windows when they put in the key. This should put the system back to a pre-key state.
uhm, sounds a bit suspicious...I can only assume (sorry oalee and CimmerianX) that you don't have a genuine copy of windows. You should buy a genuine copy of windows and the serial number will come with it. If you're trying to install the same genuine copy of windows onto another machine you should be able to transfer the licence to it.