Change primary display

Hi to everyone.

I run windows vista home premium on toshiba laptop and an external monitor.
Everything runs fine until yesterday I inserted a 2GB SD card and enabled
ready boost. The system hang and I was forced to do a brute shutdown. When I
restart I found that windows now use external monitor as primary display. I
went to display settings and change settings to make laptop display as the
first monitor. This works, but whenever I reboot the system or change user,
after login screen settings for multiple display are resetted and external
monitor became again the first monitor and the relative position of monitors
are also resetted.
This is not a blocking problem, but it's annoying to change again the
settings at every reboot.