Additionally, as of January '07, those who use Office Update will be required to validate that their Office software is legit before they can proceed with the service
Microsoft, which claims the process will be "quick and simple" for users, has stepped-up efforts in recent years in attempts to prevent its products from being pirated. It has had some minimal success in places, but has angered many users - with some in the past having their legally-acquired software wrongly tagged as illegitimate due to bugs in the system.
Source:
InfoWorld