I have two Dell PCs. And each PC has a Windows Vista disc and a Windows 7 upgrade disc which Dell gave me for free. And each disc has a product key. There are 4 product keys in total.
I'm going to sell one of the Dell PCs. But unfortunately I don't know which product key of Windows 7 corresponds to which product key of Windows Vista. If I sell the PC with wrong combination of a Windows Vista and a Windows 7, error will occur and I will get in trouble.
How can I know which Windows 7 corresponds to which Windows Vista?