What does CPUID or Speedfan report as the fan speed in RPM? Fans are a mechanical component and sooner or later they wear out or get clogged with dust. Of course, knowing the RPM isn't that helpful without knowing what it's supposed to be, so if you are OK with opening the machine, take a look at the fan and consider
1. Is it very dusty?
2. Is it spinning freely? If (while the PC is OFF!) you give it a spin with your fingers it should spin very freely, taking several seconds to stop. If it just makes a turn or two then stops, it probably needs replacing. New CPU fans are pretty cheap though, around £10 here in the UK.
As always, when opening your PC, make sure you are grounded (touch something metal, like the case) to prevent static discharge and that the machine is off at the mains.