Bear on "Another house renovation, but not professional!"

Radiators should be under the windows because they are convectors and they then circulate the warm air near the ceiling across the room and back to the radiator. Put them away from the window and you will be pulling the cold air across the floor.
As has been mentioned usually dead space too, and you also don't get convection staining under the window.
As for curtains pulled in front, or tucked behind then I'm sure that is a form over function argument. Or get blinds or the right length curtains?