retaining display settings on dual monitors

Hi,

I'm using Vista Home Premium on a laptop. I usually also connect to a
second monitor, and use them concurrently.

However, for some reason it seems that Vista often reverts the
external monitor to a lower resolution setting, and also changes the
left/right order of the monitors.

This usually happens when I do something like install a program,
install Windows updates, or reboot the computer. I don't think it
happens in all cases, however - but it happens enough that it's an
annoyance.

Does anyone know why this might be occurring and how to get Vista to
retain my settings?

Thanks,
Steve