Activation problem in windows 7

Hello Guys,
when I started my pc today & put user account password what i see suddenly before desktop a message saying that your copy of window is not genuine & when i click on ask later another window open with some message. Now when i close all the windows my desktop finaly comes with black backgroung image & also a watermark appears @ the right bottom of the window. I removed watermark through watermark removal tool but unable to activate my windows 7 copy. Can somebody help me in this matter.

Well, it seems that you are using a illegal version of Windows 7 on your pc? I suppose you havent bought the genuine copy from the microsoft website or any other like amazon and downloaded the pirated version of Windows 7 and installed it on your pc and therefore I would say that you have to buy a genuine copy of Windows 7.