I posted this problem a few days ago:

"Subject: Vista doesn't save monitor settings?

I am using a Toshiba Satellite P100-160 laptop, but am also trying to have
things set up so that when my laptop is on my desk I have an external
keyboard and the main monitor is an external Samsung SyncMaster 913N.

Because we work from home and there are young children around I have my
screensaver set to make me log back in again when it is taken off.

I can set things up fine so that the external monitor is set to my main one
etc but the problem I have is that when I shutdown/restart or log back in
after the screensaver has started up Vista switches everything back so that
the laptop is my main screen.

I am in and out of our office all the time so its getting to be more than a
pain to have to keep putting things back the way I want them every time."

If anybody's having the same problem, and is stumped on how to overcome it,
this is how we've managed to do it:

Right click on the desktop.

Choose "Graphics Options".

Choose "Output To".

Choose "Extended Desktop".

Choose "Monitor+Notebook".

Problem solved! My external monitor now stays set as the main monitor and
when Vista does its flickery-pokery on coming out of screensaver mode things
stay exactly how I like them.

Ali