How can I stop receiving the "This computer is not running genuine Windows" message?

No, I have never re-installed Windows 7 Professional. The PC came with Windows pre-installed. And, no, the system has never been inactivated. It has been in almost continuous use and always ran Windows Update automatically, always installing the recommended updates. That is, until Microsoft ending support for Windows 7. I can think of nothing out of the ordinary that could have caused this. It just started about a week ago. The computer was not turned off, it was only "asleep." One morning when I started it up, the pop-up appeared first thing - almost as if it had been there all along. This happens all the time now, not only when I start up the computer but also sometimes when I'm using the computer and also times when the computer is just sitting idle.

This is the pop-up window:

Nonsense! This is a Gateway FX computer and came with Windows 7 Professional already installed. It is a legitimate version, properly licensed, with all the bells, whistles, and blessings. I have never installed an OEM version of Windows on any of my computers. I do have the product key, the twenty-five digit alphanumeric number. If what you are saying were true, then why hasn't Microsoft questioned my use of this computer and my version of Windows 7 long before this? And why they have always provided updates up until they stopped support? No, it's only a ploy urging me to Upgrade to Windows 10. The version that I have is genuine. I do not intend to upgrade Windows. All I want to do is no longer see that pop-up window on my desktop every time I turn on the computer or leave it untended for a period of time.

That message means you have not activated your installation. The message indicates that you are using someone else's (Microsoft) software without a licence. When you bought your computer it likely came with a licence for a specific version of Windows. If it was an OEM purchase, that licence is not transferable to another computer or another version.

If you bought a licence at a retail store and installed it, you can install that licence on ONE computer as long as it is again, a licence for that version.

If you do not have a licence, it is not genuine. You must provide the Microsoft Product Key that was imprinted on the sticker that came either on the machine or on the licence you purchased in a store.

Well.... it's better. I ran the routine and rebooted. No pop-up and ran for almost three hours, then got the pop-up. Drat! So, I ran the routine again, rebooted, and ran for several hours. No pop-up. Thought it might be fixed. Put computer to sleep. Started up a few minutes ago and the pop-up appeared first thing. Oh, well.

That seems to happen to a lot of people and is most often solved by simply doing the activation procedure.

1. Put the computer online
2. Right-click on Computer
3. Choose Properties
4. At the bottom of the window that pops up you will see the activation app
5. Choose activate online now
6. IF that does not work, enter the Microsoft Product Key that came on the machine
7. IF that does not work, choose the option to use the telephone
8. Select your country to get a phone number
9. Call that number
10. You will see a very long number
11. You need to enter that number
12. AFTER you complete that, it should give you an activation code which should work
13. Write that code down because it is reusable on that computer.