Windows 7 now incorrectly showing as non genuine after Norton security install

I've just changed from norton internet security to Norton Security (NIS was about to run out so bought the new version off Amazon) and after the install Windows 7 is now telling me it's not a genuine version, it is a genuine version and was activated 2 or more years ago and has been used just about every days since with updates installed regularly.

I've tried to roll back to restore point but there isn't any, bit strange as I'm sure I've created restore points after installing windows and at various points since.

Before I go through the microsoft hoop jumping exercise and try to activate by phone is there any other suggestions, this is a work computer and it's quite old so I'm not going to consider repurchasing software that I have already paid full price for from a reputable company, I'd be better putting the money towards a new computer!

Nothing else has changed on the computer, the only thing I've done is install norton Security this morning. during the process the old version was uninstalled and the computer had to be restarted, then the install continued and registered itself with my Norton account, the install is showing properly there.

Regards
Steve