This sounds like an issue with the graphics driver on that XP machine. I've seen all kind of weird issues due to driver bugs.
Is this an older video card? See if there is a new driver available for XP. They might also want to try other display settings e.g. 32bit color, 24bit color, 16bit color.
Also try to temporarily reduce the graphics acceleration (display control panel..settings..advanced..troubleshooting) by a click or two. Any improvement?