Mike reinstalled Windows 7, but it won't authenticate. He contacted Microsoft and they want him to buy Windows 10. Leo says that can happen when modifying a computer significantly to the point that Windows doesn't recognize it as the same computer. But Microsoft still supports Windows 7, and they can reassign his serial number to the new configuration, so that's odd. It may be that using reinstall disks that didn't come with his computer could be causing it.
Leo also says to double check that he's using the exact same key. He should also try to get Acer involved. If they made a change, that could be where the hiccup is. Mike should call Microsoft again and ask to talk to a supervisor. He should just be nice about it and lay out the situation. They should let him activate it. The number to call is (800) 718-1599.