lots of white smoke from exhaust?

hi
i have always had a big puff of black smoke from my exhaust on start up but fine the rest of the journey. (people say its because i dont drive it enough)
But recently i have a big puff of white smoke when i first start and drive away. if i sit and rev it it also pours out white smoke. i recently drove about a mile and back(with a half hour break and thought i seen smoke comming from under the bonnet. i have also notice a grey slydge in my water tank (i pressume to the radiator. any help

diane
August 2009