masonry screws and brackets would work. you may have to drill holes first -- you can drill more easily into the mortar, but i'm not sure how that would affect your home. you probaby want really heavy duty brackets, though, and screws, because you don't want them falling down. what type of window boxes are they? plastic? wood? light? heavy?

they do sell windowbox brackets at hardware stores -- L shaped brackets work, too. you'd probably want to attatch the L brackets to the house first, then screw the window box to the brackets, then fill it with soil. i recommend at least two big L brackets on the bottom, and one on each side.

good luck :)
heidi