Windows 7 repeated requests for activation - 18 months after first activated!

I bought a Scan 3XS system in August 2012 and had it delivered and set up in September 2012. Windows 7 Home Premium (bought with the computer) was successfully activated at that time and has worked fine with no complaints for the last 18 months.

Over the last couple of weeks I have had repeated requests to activate Windows 7 within the next three days or lose functionality. So far I have re-activated successfully each time. But, (a) this is beginning to be annoying, and (b) I am worrying that after a certain number of re-activations I will not be able to do so any more. The request tells me that changes have been detected on my computer which mean that reactivation is necessary BUT, I have made no changes to the computer AT ALL which should lead to this.

Using Event Viewer, I notice that during March I have recorded repeated error 36885 warnings for Schanne(?) saying "When asking for client authentication, this server sends a list of trusted certificate authorities to the client. The client uses this list to choose a client certificate that is trusted by the server. Currently, this server trusts so many certificate authorities that the list has grown too long. This list has thus been truncated. The administrator of this machine should review the certificate authorities trusted for client authentication and remove those that do not really need to be trusted." and wonder if this could be connected?

I have no idea how to get to "the certificate authorities trusted for client authentication" nor how to remove them so that the list is shorter.

I have Software Protection set to automatic and started, but notice in services that SPP Notification Service is shown as manual and not started. Not sure if there is any connection with the reactivation problem.

Any help appreciated.

Thanks

John