I recently purchased and installed the P191W

Find your monitor's native resolution, in your case it's 1440x900. Go to display settings (usually right-click on desktop and choose properties), go to settings tab and move the resolution slider until it shows 1440x900. Confirm, the screen should flicker for a moment and adjust - and you should be fine.
If there is no 1440x900 resolution available, then installing a monitor .inf file that came on a disk accompanying the monitor might help.