Ah-ha... 3,4 is a memory issue. Power off, unplug and press/hold power button for ~15 sec. Open case and carefully re-seat each RAM module (How many do you have?) in its slot. Check Service Manual for instructions. Keep track of which module comes out of which slot, so each one goes back in its original slot. Then remove motherboard battery and press/hold power button for ~30 sec to clear the error. Reinstall the battery, close up, connect mouse, monitor and keyboard, and boot PC. Hopefully that's all it takes, but there's more to try if it still doesn't boot. Post back and let us know what happens...
I have a Dell XPS 410 that has not been powered on for years.  There is a Nvidia video card plugged into it that gives me two DVI slots.  I've tried both now, and I am not getting any video output? 
XPS 410, DVI monitor, no video