|Did you install the correct monitor drivers? If not then it could also be that your graphics card needs updated drivers.|
It could also be caused by the connection from monitor to graphics card.
if the interface between the ggraphics card is based on analog and the monitor is strictly HDMI then the problem could be caused by that.
Flat screens used to be auto switchable between analog and HDMI but I have noticed that more and more demand that a fully digitized graphics card be used and an adapter just will not work.
Check M$ updates/ window updates... use the custom setting and see if there is an updated driver for the monitor or visit the manufacturer website for a new driver for XP