Why is white smoke coming out of my exhaust when i start my porsche?

Why is there a big cloud of white smoke when i start my porsche boxter but it stops when i drive it.?There are no warning lights and the oil and water are fine and no strange noises.? It is a 1999 model and has 47000 miles on the clock with only 7000 miles since the last service. It has only just started doing this and was fine the day before. Dose anyone have any ideas i can try please.?