PC won’t boot.

I have a Compaq Presario 6195EA, about 3 1/2 years old, was running XP but now just a humming space heater in the corner. When booted, case, poweer supply and processor fans all run but nothing else happens. Power light on and HDD activity light on and steady, usually flickers when accessing drive. No output to screen, remains blank, tried another graphics card but no change. No beep codes heard. Checked seating of memory modules and PCI cards; all OK. Put bootable CD in drive but no joy. Is it likely BIOS battery dead, any other possibilities? Thaks in anticipation.