Is your windowframe made of wood?
When we bought our house, we noticed water trickling down the wall from upstairs. (This wasn't as much a problem as it is for you, as we had no floor at that time and the walls were bare as well
) This only happened on rainy days with a strong wind from the west. So, we checked the upstairs windows and the wood was so rotten, the water was just "pushed" through...
(needless to say, we also replaced those windowframes even though we never planned for them to be replaced!)