Intermittent High speed fan and no boot on 4700

Hi when I press the start button sometimes the fan runs very fast and my Dimension 4700 will not start.

Have checked the cmos and it is fine so am discounting the cmos motherboard battery

When system starts correctly all runs fine, fan normal speed, and cpu cool ie no overheating

Now the really wierd bit .......I hinged back the green CPU cooler to clean out dust etc and found that this could either fix, or start the problem given above.

Apart from the obvious multi cpu connections to the motherboard socket is there anything else that could cause this like temp sensor (unlikely as once the thing starts it runs fine with no overheating) or......

Could really do with some help on this one please