Windows will display "not genuine" messages if your copy of Windows is not activated properly. Product activation is necessary so that Microsoft knows that you have obtained Windows legally and to limit the number of computers using the same product key.

There are a lot more things that can happen than just error messages if you don’t fix the “Windows is not genuine” problem. If your copy of Windows isn’t activated, your computer might log you out every hour, restart occasionally, give you a permanently black desktop, or even prevent you from installing certain Windows updates.

The error varies depending on the Windows version. For example, if Windows 7 Build 7601 is not genuine, you will see this error:

You'll probably also get occasional pop-ups or see messages in other parts of Windows. Here are some examples: