I have a compaq Evo N800v which started to show problems booting and when it did boot thro post and in windows the screen would show flickering lines especially when u moved the screen back and forth, when the display was complete though it was clear and bright.

I was told it could be the ribbons that lead thro the hinges to the motherboard, so I have replaced with a new set (that were sold to me as working) but now I cannot get it to boot or post at all. The only luck I have had is when I changed the memory chips and used an external monitor it did come up with a screen to say that the memory configuration had changed, and asked if I wanted to accept the change, I pushed F1 to accept but the screen went blank and did not post at all. Could this be the video chip and maybe motherboard is stuffed??

Any ideas would be appreciated,thankyou

Try putting the original memory back. Plug in an external monitor.
On my laptop, you have to press a couple of keys to toggle between laptop screen & external monitor.

Before posting try google. Backup. Use anti virus software.