Not knowing the brand or model of your monitor, I can’t give you a specific solution. XP being from before the wide-screen-rage, you might have to change its setup and change the ratio manually, by using a utility called “display properties” where you can select different aspects.
I bought a Samsung 21.5” LCD monitor last month, and am noticing that the image slowly shifts randomly to the left or right a few pixels. First I suspected it was because I was still using the VGA input (with the supplied cable, so I suspected that should work fine). I then bought an HDMI cable (6ft or 1.5m) + DVI-HDMI connector, and still have the same problem.
Compounding the confusion is that, just clicking on the configuration and ON/OFF button - without changing anything, the image is again restored to the screen center. So the problem is definitely with the monitor.