CPU upgrade problem

Hi all,

I'm having trouble upgrading my machine from a PentiumD 925 to a PentiumD 945.

The chip goes in fine but 99 times out of a 100, the CPU fan starts and then stops and then I get a red message on the screen along the lines of:

"CPU fan failed. The machine will shut down to avoid CPU damage."

The fan itself is fine, if I put the old chip back in it works as normal.

I've seen advice on forums elsewhere about setting the fan/temperature tolerances but I don't seem to have any settings like that in my BIOS.

Presumably the problem is with the heatsink and fan. I noticed that the heatsink got hotter and hotter until eventually the machine would power down as soon as I turned it on.

Is it worth trying to get a better heatsink and fan or am I wasting my time? The chip came with a heatsink and fan but this doesn't fit my motherboard - it is a clip in one whereas my existing one screws in.

Any advice folks?