If I read the directions correctly (We all know how well I did

previously

), the Key is your OLD W7 or W8 key from the COA Label on your PC.

Do keep written copies of this in a safe place, the labels suffer on portable devices. Watch out for O vs Q and similar
Unfortunately W7 COA Barcodes won't produce meaningful (text) when scanned with a real bar code scanner (XP bars scans just fine)