Switch default monitor in XP?

I have an 15" i5 MBP and I run XP on it for some games. I like to run most things on an second monitor but when I play games full screen it defaults to the display on the MBP. How do I mirror my displays or make my second monitor my main display?

Is it even possible?

 

That works great when you are not running it on an iMac or MBP. The option to make that display your default is greyed out.

Has anyone made this work?