This is correct. The reason that radiators and floor vents are located directly under windows is because condensation and heat loss are most likely to occur there. If the floor vent is pushing hot air toward the middle of the room, the windows are not exposed to enough heat. Either turn the vent controls so that the air shoots straight up, or if the vent has a deflector tent over it, remove it. The second thing that you can do to provide more heat to the window area is to move the blinds in from the window as much as 15 cm (6 in.). Most people when installing blinds install them flush with the window. Again, any heat rising will bypass the window and rise to the ceiling on the room side of the blinds. Eliminate those two problems and the condensation should stop.