Cert of Authenticity is like a "receipt" from microsoft as purchase proof that the key is legit. IF you buy a key from ebay, the guy would have easily wrote down his company's corporate key and sold u a disk that the company had lying around when they got 300 dells. It would work fine becuase there is no activation procedure or anything so you wuold never know its fake unles microsoft flags it in thier "genuine advantage." The only way to ensure that you weren't cheated is if you have an actual COA to show the cops when/if they knock on your door. The sticker is to be affixed to your case at all times.
Second, a key oem limitation is that it is bound to one computer meaning you can not move the key from system to system even if you completely wipe the old system. Retail allows for key moval. However, microsoft support is rather lenient on this one and if you say something like the board blew up they will let you have it again.
Second, a key oem limitation is that it is bound to one computer meaning you can not move the key from system to system even if you completely wipe the old system. Retail allows for key moval. However, microsoft support is rather lenient on this one and if you say something like the board blew up they will let you have it again.