It's stored in the system BIOS, and is automatically accessed when necessary. You can recover the number using a utility such as Belarc Advisor, but you probably will never need it.

I need to find my Product Key because I am getting steady messages from Microsoft asking for the Registration number. I bought my computer from Dell, North York, ON.  I don't recall where I saved the number. I assumed I wouldn't have to reregister it. 

Systems that came with LEGIT WIN8 OR WIN10 key in bios will not need anything.

OLDER models that had XP/VISTA/WIN7 will be asking for a key.

You will have to be online every 180 days or windows will DE ACTIVATE.  Nobody at Dell or here can issue you a key for windows.  That comes from microsoft or from COA sticker on the system or from within the bios for WIN8 and WIN10 machines.

error code 0xC004C003 Product key is blocked by microsoft is very specific and means you have been sold a fake key.

Error code Error message Activation type