You may be able to audit the currently-installed Product Key as suggested but I am not sure it will work if the factory OEM-SLP activation method was used to activate the current OS.
Using the Product Key on the COA sticker requires a different OEM-COA activation method for which only the COA key can work.
In the past I have been able to have MS phone activation parse the key read from the HD by a program like Belarc Advisor, however others report that it won't work.
For this reason your best course is to pursue replacing the key with the manufacturer, or ordering the manufacturers Recovery Disks which will self-activate. Of course you can do this while concurrently trying a clean install with SLP key read from HD, but no promises.
Bill2 will correct me if I've gotten any of this wrong.