I have got it...
When I had installed Office 2007, I did not enter the Product key, but instead modified a proof.xml file. Yesterday when I was updating windows, Office 2007Service pack 1 was installed. So I think this service pack disabled my office 2007.
Now I again modified the proof.xml file to its original entry and then when I started Ms word it asked me to enter the product key, which I did, and every thing was back to normal.