Problem of Vista display setting at computer start up

I have this problem with Beta2 and RC1. No problem with XP.
When starting my computer, after the initial text screen, the computer is
supposed to display the dual boot options for Vista and XP. Now this screen
cannot displayed on my monitor correctly. It is also the case of I let it
get into the option screen for choosing safe mode, etc... But after that
when windows is fully started the screen is fine again.

My monitor (also an TV) accepts 800x600 60hz, and couple of other setttings.
When the display output does not output these settings, the monitor cannot
display correctly. So somehow at this stage Vista set my display into this
mode that the monito cannot display.
Before Vista get into this mode, the display is fine with computer bios
starting process or bio settings.
This problem does not appear with XP.
Why Vista change this setting mode to something else for this process,
namely, when choosing dual boot options or safe mode selection setting
display?
(I cannot see these selections but I try to guess what's displayed and make
selections like a blind person, to proceed in to the full Vista mode ...)

Shawn