hi, could anyone help, i switch on my computer and after about 3/4 hours my cpu fan starts and is quiet loud it comes on and then goes off quite often, when it starts the computer freezes and when stops it is fine,and whist its goin on and off the computer slows down. i have a 1gb RAM and 250 gb hard drive so i know its not them iv checked my temperature on it and its between 30 and 40, which is fine.

You can replace the fan for as little as $12... I would get a complete assemble with CPU fan, heatsink, and thermal paste... which may cost anywhere from $12 to $25.