|"why would it be doing this when the product was registered when i set it up 3 years ago?"|
Registration of Windows and validation of Windows are two different things.
Registration is optional, whereas validation/activation is compulsory. There is no requirement to activate pre-installed versions of Windows however, since it's already done at the factory.
What may have happened is someone else is using your Windows Product Key, and when that happens, Microsoft "blacklists" it so that even the genuine owner of that key (you) eventually receives the "not genuine" warning.
You will have to contact Microsoft.