imac multiple monitors

Hi

I have a weird startup problem on my iMac 27". It boots past the apple logo and blue screen but then I getaway white screen with random grey checkers all over the place!

I had issues with my snow leopard install which we're fixed after a fcsk command.

For some annoying reason I can't boot to a different USB or flash drive as I don't get past the mac logo!

Any help would be appreciated