"Answer for XP: In safe mode, you won't be able to change the resolution (or set refresh rates). Safe mode boots XP using a generic VGA driver, not the graphics driver you have loaded; this allows you to recover from video-drivers-gone-wrong type problems. Instead of booting into safe mode, you'll need to boot into VGA mode. This isn't very well documented as far as I can tell; as best as I can recall, you'll need to do the following:
Hit F8 twice right after the computer boots. You'll see more options than you'll typically find in the normal F8 boot menu.
One of the options is VGA mode; select that option. XP will boot like normal, except it'll force the video card to boot at 640x480 with 4 colors @ 60hz. From there you can change the display settings however you want. You won't even need to reboot to get out of it."