Question : Problem: CPU fan running to fast PC freezes

when I boot this HP Proliant pc the CPU fan initially runs very quickly (boosts) then settels down to a normal running speed. after the PC has been used for while (20-30 mins) the CPU fan boosts again and doesn't slow down. PC Freezes. I assume this suggests the PC is overheating. I have:
1. run Micro 2000 diagnostics on the unit, this has found no errors.
2. exchanged the graphics card: this seemed to improve things for a time, but now the unit keeps freesing again
3. inspected the fan: all seems OK
The only common applications running when this happens is the internet or word
OS is XP Pro
No error messages received
Can anyone Help?

Answer : Problem: CPU fan running to fast PC freezes

CPU will use 100% of the CPU generating maximum heat!

Sorry, when I say 'jump' into the BIOS I mean just enter the BIOS (maybe F2 on HP) - you will see an entry somewhere in there to view temperaturs etc...