One of the interesting aspects about this fascinating novel is the way that windows are used to symbolise the way that Charlie is separated from those around him. A number of the memories he has when he becomes intelligent involve Charlie looking through windows, which indicates the way that emotionally Charlie is so separate from others. Note the following example as Charlie is just about to have his first journey in a plane:
Through the window--in the clouds--I see Charlie. Age is difficult to tell, about five years old. Before Norma...
The window therefore seems to represent all the ways in which the emotionally retarded Charlie is kept separate from those around him, and the ways in which he is unable to engage in life. The symbol of the window makes us realise that Charlie is a passive observer of so much of life: he doesn't understand what he sees and he is unable to participate.