Download WOLF from my site:
http://www.bytemedev.com/wolf/
Run the program.
Go here:

Easy peasy.
RC
Old Way (Click to show)
Quick trick, the key is stored in memory under the ACPI/MSDM Tables.
Now reading it is the hard part, eventually WOLF will do this for you no questions, but until it does, it requires a memory reading utility.
For this, I used
RWEverything Utility .
The author has made a very nifty program.
Instructions are simply, when the program loads for your OS version (32/64-bit), you click the ACPI shortcut up top.

Then choose MSDM. If your key is available, it's in a simple string.

Note: You can still be unlucky in the fact that Windows 8 uses more letters in its product keys than Windows 7. IF you are downgrading and get the invalid character message, you have to contact MS to get it activated.