If your Windows Product Key label is worn off or unreadable you can forget about getting any help from Dell or Microsoft. You are basically on your own. Microsoft will send you to Dell. Dell will send you to Microsoft. Microsoft will try to charge you money. Dell will try to charge you money. You will spend an hour chasing your tail and you will get nowhere.
It's a very simple and very common problem. It is bewildering to me that there is not some sort of procedure in place to deal with a worn off or unreadable COA label.