Problem with graphics/screen

Hello

My computer has suddenly not wanting to give me access

I boot the computer and the apple logo comes up with the spiraly thing and everything is fine, then i get the cursor and I can move it, as soon as I get to the login screen after 1 second the screen goes black, I can still login and play things, I just can't see anything

I've tried all the troubleshooting on bootup, reset the VRAM and trying safe mode, always the same problem, booting from a liveCD or snow leapord dvd does the same thing

Does anyone have any ideas what's causing this? Seems strange that graphics work fine on bootup but when I get to something i can use everything goes black!

Any help?
I'm running Lion by the way
Thanks