Are you asking how get the current, in-use key that your machine is now using? or did you loose your key and can't install Windows without it?
If you are seeking the in-use key, get SIW (System Information for Windows) free at
www.gtopala.com
The System key is displayed by clicking "Secrets" in the left-pane of the program.
If you lost your key around the house, do as soundguruman said in the reply above this one.