Replaced Screen, Windows Xp Logo briefly appears then nothing?

Hi.

We have had a compaq r4000, where we have replaced the screen, cable and inverter.

When we power on the PC, we see no bios/starup text only the windows xp logo for a short period (looks in limited colours also) then all goes black.

If we connect a monitor to the vga port the PC seems to work okay, with all bios screens and windows loading properly.

I have tried updating the bios (just to rule it out), resetting the bios etc. But am now stuck on what else we can do.

Tried putting in the old cable, screen and inverter in different combinations and still no luck.

I would think it was the graphics card if, but then I cant understand how the external screen would then work.

Now banging head against hard door