Windows in airlines passenger cabin is Oval and not Rectangular or Sqaure.  

The size and shape of the windows can weaken the fuselage structure. The large windows with square corners could cause a fatigue crack to develop in the sharp corner of the window. So the windows are small and oval in shape.

Hi sreejithsreenivas! I think it has something to do about the stress and strain of the plane structure when going against the wind. But I am not really sure how to go about the explanation. I just remember this being discussed in one of my engineering subjects and I can't recall the exact explanation of my professor. Sorry, having a memory lapse on that one!
Take care and blessings to you!
lovelots..faith