another monitor on this computer?

If this monitor tests OK on another system and the drivers are up to date, I would suspect the graphics card.

But before going that far, a few comments;

When you updated the drivers did you reboot the system afterwards? If not, then try that now.

Also, when did this happen, and what occurred when it did? If it was in fact a driver update, try rolling back the driver.

Has this Pavilion ever had the case opened and the dust bunnies blown away from the fans and the vents?

If nothing helps, then I think you are looking at a failing graphics card and since it is integrated there is no replacing that, but if the system has a spare PCI slot you could add a video card to it. In fact the CNET video review in the link below suggests that is possible;
http://cnettv.cnet.com/hp-pavilion-a6400z/9742-1_53-50001892.html

Mark