Attempts to start when mains cable plugged in!

I have worked on macs for a long time but this one is new to me. Got a 17" Intel with a 'lines on screen problem', so I acquired a suitable laptop screen and an adapter from realmacmods. Came to fit it - didn't work - fans blowing fast - white LED on. So I refitted the old screen - same result.

Then I reseated the RAM, reset SMC more than once - same result. This is where things get really weird. After leaving it for a few days to think the problem through and do a bit of reading I tried again. Now as soon as I plug in the power cord the mac tries to start up without any power button press! It still has fans blowing fast and white LED on. The 3 green led's on the logic board show trickle power is OK and that the logic board has detected proper power from the power supply, and that the computer and the video card are communicating.

I can usually fix mac problems, but this one seems to have got me beaten. Any suggestions would be welcome.