(ignore the "Graphics Cards" sub-category - this forum interface forces me to select something)
I'd been running with optimal resolution for a long time, but after a recent hard-restart I no longer have a 1920x1080 option in display settings. This makes me suspect that windows updater changed something between my two most recent system restarts. I've already made sure I have HP's latest driver for the 2509.
Try uninstalling the graphic driver and either re-install the same version or download the newest version.
I supposed I experienced something similar to what you are experiencing a couple of years ago. My 1920x1200 resolution monitor stopped displaying at 1920x1200 clearly; all of the sudden images would look distorted for whatever reason. Dropping the resolution to 1920x1080 cleared up the graphic, but of course I had black bars on top and on bottom.
I kept it like that for about 2 months while I tried to resolve the issue. Re-installing the driver or install new ones did not work, scanned for malware or viruses, but none were found. Detached the DVI cable, then re-attached the cable. Re-seated the graphics card (pull it out, put it back in)... nothing. The last resort was to re-install Windows XP. That fixed the problem.
What was the problem? Not sure, perhaps the Windows Registry somehow eventually became corrupted perhaps.