Dell Latitude 820 power up issue

Hi, I have a D820 laptop which was running fine until today. I inserted a DVD into the drive to install some new software and the machine totally locked up at the first screen of the installer, mouse cursor,frozen, keyboard non responsive etc. Ultimately had to resort to holding the power button down until the machine made a forced power down. Machine was operating via a 90 power adaptor and had a fully charged battery in good condition.

When I went to restart the machine, it shut down at about the end of POST, no beeps or flashes ftom the caps lock, num lock, scroll ? LEDs. Repeated the power on and observed:

Green power light and blue bluetooth light, three green lock indicator LEDs solid for about 3-4 seconds then entire unit powers down again.

Have tried repeating with battery power only, mains power only,  holding Fn and powering up, disconnect power and battery and hold power button for 15 seconds, reinstate battery and mains adaptor, always with the same result. I have consulted the POST indicator light sequence interpreters but they don't deal with my situation.

I suspect that my machine has died, just after I ordered it an SSD and memory expansion but before either was installed. Has anyone got any suggestions other than buying another computer?

Thanks 

Mal