Laverne, Go to the link below and under the letter "T" scroll down to "Themes (Won't Load or only Windows Classic is listed)" where you'll find several fixes. The Windows XP Theme is called the Luna Theme.
Files from the above link are Safe to download & run.
I just noticed that I can no longer change from Classic Theme to XP Theme. I've tried everything suggested in the forums for the last couple of days and still no luck. I can go to the Themes tab in Display and click to the XP Theme, but when I then go to the Appearance tab, the only choice under Windows and Buttons is Windows Classic style. Also when I close out Control Panel then go back in again to Display, it says Modified Theme instead of XP Theme. Thank you