This error message comes if you use pirated or OEM versions of Windows 7.
If windows 7 is not genuine, even you can't change the background wallpaper or any other display related option on your PC.
So now we are going to show how you a easy way to make your windows 7 genuine using command prompt.
Make Windows 7 genuine using Command Prompt
Press Windows key + R
Now "run" box will appear , in "run" box type CMD
Click on OK
Note : Must run CMD as Administrator
Now command prompt will appear
In command prompt type SLMGR -REARM
Wait 5 to 10 seconds.
Now a message will appear as shown in image below.
Everything is done !!!
Now your Windows 7 is activated.
Restart your PC once. Here after you won't see the warning message that "Windows 7 is not activated"