[MetalMilitia]

Probably just a problem with the power saving profiles. You can manually set the brightness on mains power here:

Control panel -> Power options (possibly slightly different in XP). You also might find that you have a hot key for adjusting screen brightness on your keyboard.

To permanently fix the problem you may just be able to remove the HP power saving profiles application (which no-doubt exists) and handle any power saving modes with the default windows application.