White(ish) screen?

[AJS]

Hello all, I'm after a little help if possible.
Last night I was working on my iMac when it froze (no response from mouse or keyboard) forcing me to restart it from the power button. Upon restarting it presented this screen:

I can start up in safe mode without any problem and all is working ok but am greeted by this screen each time I start up normally.

Any help with this would be gratefully received (as I've a ton of work to get through!).
I'm running OS X 10.6.8

 

[Cory Cooper]

-Which exact model iMac?
-Can you boot from the install CD and does the screen do the same?

C