What registry key or windows file determines which monitor are set as primary one in a multi monitor environment?

What registry key or windows file determines which monitor are set as primary one in a multi monitor environment?

HDMI monitor is set as the default monitor, and another VGA monitor is used as the secondary monitor to mirror my desktop, the issue is: when the HDMI one is plugged out, xp system will set the VGA one as the primary monitor even the HDMI is re-plugged in, and reboot does no help, the VGA is also as the primary monitor.