Actually I rebooted the computer and the message disappeared, and there is now full information on activation and the product key (i.e. it IS genuine!) But it gave me a nasty shock for  a while. I wonder if a Windows update had messed up with the record of activation temporarily?
This message suddenly appeared on my laptop this evening. It is a Dell UK 5030 laptop bought with Windows 7 preinstalled so it has to be genuine. Any advice please.
"Windows 7 Build 7601 This copy of windows is not genuine"