My Dell Latitude C400 PC Notebook screen flickers very fast, any ideas? thanks

It can depend on what kind of flickering you are seeing. A very very fast one, that shows more obvious if you look to the side of your screen (instead of directly at it as normal), means your screens 'refresh rate' is most likely wrong.
That's the most likely problem you have, as the inverter will usualy only make a screen flash, not flicker, though they don't usually flash too much before the bulb or inverter can't take the strain and breaks properly.
Sometimes a screens circuit can go slightly faulty (very rare) and cause a slight change in its refresh cycle, and that can cause a high speed flickering too, usually accompanied with a very slight blurring of any images.
The standard setting for a C400 is 1024 x 768 resolution at 60 Hertz, so check your display properties to see that's not been accidentally changed.

Try an external monitor to see if it's the screen. If it still does it, the motherboard is bad. If it does not flicker,try replacing the inverter for the screen first,then the screen itself if that doesn't fix it.