Too bright in what way? Color choice? Try a different color for the XP
theme in Display Properties> Appearance. Choose from blue, green or silver.
Or if using Classic style XP, change the Scheme from the Advanced windows
for the Appearance page.
If you are talking about a monitor that is too bright overall, try using
the controls on the monitor to adjust. Most will have adjustments for
brightness and contrast, similar to adjusting the same qualities on a
television set.