If its a genuine OS SP3 then the product key should be available to you via these simple steps
Click Start, and then click Run
Copy and paste, or type the following command winver click OK:
A dialog box displays the version of Windows and the service pack that is currently installed on your computer.
Note If you still are not sure which service pack that you have, you might want to ask someone for help, or you might want to click
here to contact Support.