I think I figured out what is going on. The temperature senor for the mobo I have is very close to the video card which is 8800GTX. The card itself is hot, and it heats the sensor, I think directly through radiation, not air convection (I can feel the heat of the video card by my hand). I have overclocked my processor by 30% (2.4GHz->3.12GHz) without changing FSB😀RAM ratio (3:4 in my case) and the temperature did not change. More over, if I open the side of the case and blow the air by fan into any region of the board, the temperature changes by 1 or 2C. So I am quite sure that it is just thermal radiation from the 8800GTX.