I have suddenly started getting the error message and exactly the same time there is now a loud fan noise on the left side of my Mac Book Air. I have tried the safe boot mode I have seen on other threads and the message has gone but the noise is still there. Everything also now loads really slowly. I can't get a genius appointment until next week so if anyone can help that would be great!!
Try resetting the SMC -- Apple recommends this if you are experiencing, among other things, fan and power issues, as well as slowdowns: