I have a win xp media center validated for over five years, and since a recent update *(at microsoft)after rebooting I got the must activate screen. However, with a valid product key Microsoft can not explain why this has happened. this has been my old faithful pc and I've never had this problem. Help/; microsoft keeps giving me the run around.
Have you simply tried to enter the key that should be on the side of your machine? If that doesn't work, and provided that you do in fact have a legal copy of Windows, it may be possible to start up in Safe Mode and use a product like Key Finder to determine your product key, unless it has somehow been removed from your registry.