Windows not genuine popup, countdown to black screen

I have Windows 7 64 bit home premium, paid for with my Novatech desktop pc. As it is the 64bit 7, it arrived on a disk. It worked fine for over a year. Suddenly a large pop-up appears several times a day telling me my Windows is not genuine. I suspect this was after a Microsoft Windows Update.

My Windows is genuine. I have held off going through the activation process that the link on the popup leads to, in case it leads to my OS shutting down. Apparently if someone else has pirate Windows that used the same IDs as my legit one, Microsoft will charge me for providing new ID, key, whatever.

I uninstalled all the recent Microsoft Windows updates. No difference.
I tried system restore but it refuses to do this. Error 0xc0000022.
I tried several fixes found online. Make no difference.
There is a countdown from 30 days (now at day 28) on my Windows 7 license information on the computer.

Has anyone else met this problem?

Microsoft would not charge the legal owner of a product key for the actions of Piracy. The one thing the original owner would have over everyone else is they would have the COA ( The Sticker that contains the 5x5 product key ).
To be honest, many reasons exist as to why a system may ask to have Windows reactivated and are far more likely than someone pirated my key.

If the system is as you describe activated over a year ago with no significant changes then you should just be able to go ahead and complete activation again without any issues over the internet. If you have made any recent hardware changes it might require you to activate with the phone activation. What I would not do is start applying lots of changes from the internet as its possible this could make other problems. At the moment your system is just asking for reactivation so you should just proceed and complete this action rather than try and work around it.

I found the Windows 7 disk, in a hard case. Looks as if we discarded the paper one. I have the product key. But have lost the Novatech invoice. Before taking your advice I am going to prepare an old laptop, download new antivirus and a Linux OS for it, so I have an alternative machine to use. There are people on fora who say they have been charged by Microsoft for a new key despite their having the legit version.

Ok well let us know how you get on.

If Microsoft charged those customers it was most likely they were using the licence they had incorrectly. One of the most common is taking OEM licence keys from an old PC the customer is getting rid of to use a new PC they have just got. OEM licences do not have transfer rights from one PC to the next and as such a customer who does this would have to buy a new licence as they are in breach of the terms of the OEM Software licence.