Windows pose a unique security challenge. All an intruder has to do is break a window, reach inside, and unlock it to gain access.
There are some easy things you can do to fortify your windows, starting with the glass itself. Regular glass is really easy to break. Laminated windows are ideal, or a special kind of glass that's similar to car windshields. Installing a second layer of glass helps, as well as ensuring that panes are securely attached to the window frame.
Also, make sure that door locks are positioned farther than an arm's reach away from windows.