When i change my resolution from what was set when backtrack 4 finished installing my primary lcd monitor stops displaying anything and i can only see backtrack on my secondary crt. (they are in mirror mode)

I was wondering why this happens and can i fix it?

Also can i disable my secondary permanently or extend the desktop?

Thanks for your time

NOOBshoes

ps. im reasonably new to linux but im a computer technician so i can usually work things out