Which classifier for window (窗户) is regularly used? The dictionary lists two classifiers for window (窗户) 个 and 扇. Are both of these in common usage? If so is there any difference in the register? Does one sound more formal, etc?
一扇窗户 refers to a window on the wall, while 一个窗户 refers to the object (frame and glass) of the window.

In most cases, 一扇窗户 is the only correct choice, 一个窗户 should not be used even in everyday speaking.

However, 一个窗户 is more appropriate when:


the window is not installed on the wall yet.
the window refers to something in computer interface (窗口).
the window refers to a car window.
the window (窗口) refers to a counter (in a bank etc.).