How can I letterbox games?

My laptop is an Inspiron 3000 series, with somewhat cruddy graphics (Intel HD 4400), so to play games at better framerates, I often lower the resolution. However, recently games I scale down to 4:3 haven't been letterboxing (I suspect it's because of the graphic driver update), rather, they are stretching to fit the screen (obviously, this is undesirable). If you know how to make my games keep letterboxing, please tell me.

Thanks.