Look in your bios for any fan settings, there should be some form of fan control, the new uefi has quite comprehensive menu's but even older bioses should have something,
if no joy there then download speenfan and control the fans startup and target temperatures through that,
i would advise cleaning the heatsink though, don't just blow air into the case, take the side off and grab a toothpick, pick any chunks of fluff out of the Hs,
if its really bad then remove and wash in a sink, dry properly and replace using fresh thermal paste
but the fan should be running as soon as you fire the Pc up, and ramp up speed as its needed
Moto