Desktop background has vanished and is fully black and “windows not genuine”

Hello,
I had formatted my ACER laptop 2 days back, but now, the desktop background has vanished and it is fully black with just the icons! On the extreme right side bottom of the desktop, this message is seen permanently – "Windows 7, Build 7600, This copy of Windows is not genuine."
Can you advise me as to how to come out of this? Is this problem something serious? I am not much knowledgeable about computers. Should I again take my machine to get formatted? Is there any simple solution, which I can manage by myself?
Hope, experts like you would surely help me.
Thanks and good day.
