A COA, or “Certificate of Authenticity” is a sticker on a computer system that has a 25-digit license key for the Windows Operating System, such as Windows XP or Vista.
The COA verifies the validity of the operating system installed confirming that it is genuine. With the COA attached to the product, a legal copy of the operating system can be acquired from the original equipment manufacturer if the original restore media is lost (ie. Dell, HP, IBM, etc).