Are you sure this only happens when you turn off your monitor? Have you tried it with turning off the monitor after the PC is off? If that is the case, just leave the monitor on till the PC is off. Not worth messing with the system to fix this otherwise.
Did you re-install the drivers for the computer after installing Windows? The loss of the monitor signal should not cause the PC to reboot, but I've seen hundreds of odd things that should really be impossible.