Monitor's native resolution unavailable - please help!

The "Generic non-PnP Monitor" driver that comes with Vista is causing major display errors, such as not letting me use my monitor's native resolution, 1440x900). However, whenever I try to update the monitor driver with the one for my Acer X173w monitor from the official Acer website, it says "Windows has determined the driver software for your device is up to date" and won't let me install the monitor-specific driver.    My system is currently setup with dual monitors - both are the same model and one works fine.  Graphics drivers are loaded correctly and I am connecting through VGA.  Any help?  Thanks so much in advance.

What's your graphics card?

I'm with @SeriouslyNow in thinking the problem is most likely your graphics card drivers rather than monitor drivers. Are you running Forceware 71.90? If you are and you still can't get that resolution, you may have to create a custom resolution. There is apparently some way to do this in the nVidia drivers although I don't know off the top of my head what it is.

Look I hate to be bearer of bad tidings but you're gonna have trouble running that card in dual head in Vista.
 
I did find this post on Guru3D.  It refers to 96.85 Vista 32bit and 64bit specific drivers for Geforce 4MX cards.  It's unsupported obviously.
 
Install those drivers first and see how you go.

A GeForce 4? Man that takes me back. 
 
Used to play battlefield 1942 on my MX440. those were the days :D 
 
Incidently, MX440 + vista is likely your problem.