Compaq presario 6000 wont boot

Can anyone tell me the likely cause of this problem? .I have a compaq
presario 6000 about two years old , I have added a graphics card and
tv capture card with no problems until now , The problem began with
the monitor not powering up , it then came on and the computer
reached the xp logo screen which then turned off , after this i got a
sort of clicking sound from the hard drive ,the monitor stays in
standby and the computer doesnt even attempt to start also when i
initially switch on after a couple of seconds it turns off and then
back on again and i get the num lock led flashing on the keyboard .
Nothing seems to function and i cant even access the bios.