You might be low on coolant or have air in the system somewhere. Check your coolant level and if you need some start the car with the radiator cap off, take some water or coolant and fill the radiator. While doing this make sure the fan is on all the way and the heat is on, after filling go check the heater in the car and see how it is. If it hasn't changed there could be more air in the coolant system or there could be a problem with your HVAC Unit which could be an electrical problem(read: take to a technician)
<img src='http://www.european-forums.com/forums/uploads/post-1-1107198379.jpg' border='0' alt='user posted image' />
Formally known as Asio.