Got power but nothing else

I went away for all of last week and shut down my pc in the usual way
and also switched of the power at the mains as well. On my return
when I switched it back on, it starts up as soon as I turn the mains
on at the wall socket. The power pack starts, the fans on the power
pack, cpu and motherboard run, the keyboard and mouse light up, the
dvd/cd drives light up but it does nothing else. No beep, no hard
drives, no signal to the monitor, no response from the power button. I
also can’t shut it down at all other than by the mains on off switch.
I have removed every lead and plugin component from the motherboard,
including the bios battery, and one by one reconnected each trying to
power up each time but I just get the same thing.

I seem to be getting power to the motherboard Gigabyte GA-81PE1000
with P4 3.0Ghz proccessor as the fans on the board run but I am now
out of ideas.
2mb ram
Windows XP pro
Homebuilt pc in Lian Li tower case 7 years old