anyone got any ideas why when i turn on my pc after its been of for a while, the fan go's absolutely crazy? It must go at least 3 times its usual speed. It all youst ot be normal untill one day i switched it on and the fan lost sanity. After a while though it finally moderates itself and is normal again. Is this causing any damage?

get a new fan.

Which fan are you refering to?

The fan at the back of the pc, that pushes all the air out, that one, i dont know the technical name of it.

In that case it sounds like your PSU is defective and is overvolting the +12v output, thus causing your fan to spin faster than normal. If this is the case, then other components will soon be damaged by this behaviour. You could lose all your hard drives and other attached devices. You should check the output using a voltmeter, or looking at the voltage reported in the bios, while the fan is acting abnormaly.

If that fan has speed control, not sure if that one would or not. Seems if it did, you'd know. Anyway, if it has speed control and that is going bad, it could be the cause. It could be shorting the controller circuit and running at full speed intermittently. Nic's suggestion is also likely and you should check the +12v outputs to make sure. +/-10% is within tolerence, so if it reads 13.2v(for example), don't panic. Another likely cause could be that the fan is defective and intermittently drawing too much current.

Either way, if the PSU isn't the cause, replace the fan. If the PSU is the cause, replace the PSU.

thanks for the help guys.