Well if your game is switching resolutions to a mode that your monitor
doesn't like, it'll usually give the error message "out of range"
meaning your video card is sending a signal to the monitor that the
monitor does not support.
Might try going into your display settings and test out all the
resolutions, make sure your system can switch to those resolutions
correctly.
I searched for your monitor here:
http://www.acerpanam.com/synapse/forms/portal20.cfm?
website=AcerPanAm.com/us&siteid=7293&areaid=7&formid=3404
&LINKTEXT=Specifications
Couldn't find the AC173, only the AC713, which is a 17" CRT monitor that
only supports up to 1280x1024 @ 60Hz, if your computer is trying to
display a resoltion higher than this then the monitor will give that
error message.
I searched for Gears of War, Change Resolutions, and found that there's
an .INI file at: Documents\My Games\Gears of War\Config. That's for XP,
might be in a different spot in Vista, use search for wargame.ini
Open this file and look for the resolution settings somewhere in there.
Try setting it to 1024x768, just to get into the game, once there you
can change the resolution to whatever you want.
GL,
A.