Heater blowers too weak in Citroen Xsara Estate 2002?

Hi there

Just bought a Citroen Xsara Estate 2002 and found the heaters were blowing very very weakly. All the speeds seem to work, the temperature is working and the air conditioning but it's all SO weak and I'm struggling to de-mist my windscreen. What can I change or check myself (bearing in mind I know nothing about cars!) before I have to spend a fortune for a garage to have a look at it?