Computer fails to start up

Question

I run a HP 6000 Pro Desktop computer.
Upon switching on the powerlight turns on and stays on (fixed)
No HD reading or writing
No cursor on the screen
However after about 10 seconds all ventilators rev up to maximum speed and stay at this speed until the computer is switched off by the on / off button
Power led gives no POST warnings and no beeps are heard
Hard disk replaced with other similar computer, starts up normally, HD OK
Suspect motherboard, however no blinks of the power led
Possibly Power supply, computer clean and just three years old.
Appreciate your suggestions
Regards

Answer 1

Dear Erico
Thanks for quick response.
I have tried both your proposed options;
Firstly disconnected HD (both power cable and SATA cable) no difference,
Cleared CMOS, no difference.
Checked again seating of memory found OK.
Removed an additional Network Card from the PCI connector, no difference.
Removed connections to CD Rom, (both power cable and SATA) no difference.
Removed for sake of completeness battery, no difference.
The start up sequence apperas quite unusual:
Upon switching on by the power button, the powerlight switches on (green) and the HD light switches on for a short moment (Only while pushing the power button)
Ventilators start running on normal speed.
Access to CD rom (trying to read from CD rom) (also with a start-up disk the computer will not boot)
HD appears to rev up (I only hear the sound of revving up, however no read or write indication (HD led stays dark))
Computer shuts down automatic after about 5 seconds. (ventilators stop, power led switches off)
Computer powers up again automatically after a delay of about 2 seconds.
Same starting up sequence as above.
After about ten seconds ventilators of powersupply and chassis speeds up to maximum revs.
I am not sure if the ventilator of the processor also revs up, I tried same with the processor ventilator disconnected, no difference.
This continuous untill shutting down by hand (pressing power button 4 seconds)
While pressing the powerbutton the HD led lights up.
(power led lights continuously during start up, no POST warnings, no beeps, no cursor on screen)
It appears to me that the computer thinks that it is overheated and accordingly wants to cool down the chassis by additional ventilation. However I anticipated that only the processor temperature is monitored.
The problem first occurred after switching on at 08:00 in the morning after being switched off the day before at 17:00.
your further suggestions are appreciated.
Regards