The fans in my MacBook Pro (2007) run at full speed for no reason?
Trouble with fan speeds can almost always be solved by resetting the System Management Controller, or SMC.


  After performing normal troubleshooting, these symptoms may indicate that an SMC reset could be necessary.
  
  Fans
  
  
  The computer's fans run at high speed although the computer is not experiencing heavy usage and is properly ventilated.
  


Source: http://support.apple.com/kb/HT3964

The procedure for laptops with removable batteries is as follows:


  
  Shut down the computer.
  Disconnect the MagSafe power adapter from the computer, if it's connected.
  Remove the battery.
  Press and hold the power button for 5 seconds.
  Release the power button.
  Reconnect the battery and MagSafe power adapter.
  Press the power button to turn on the computer.
  


Source: http://support.apple.com/kb/HT3964