One possibility, Dionysusigma, is that it is an OEM copy, but it does not say so on the sticker. Is the target PC a major manufacturer's machine? Dell, HP, eMachines, Gateway, etc.? If so, the COA is probably an OEM version.
If it's a custom box someone put together, then it may well be a retail box version COA.
You won't lose anything but a little time if you try to install using the wrong version. There will just be an error message when you try to enter the product key, and it won't let you go any further. Then you will know you have to get the other type of install disk.