When you attempt to change your desktop theme, you cannot, and you cannot select the Windows XP theme.
This behavior will occur if the Themes service is stopped.
To configure and start the Themes service:
1. Start / Run / Services.msc / OK.
2. Right-click the Themes service and press Properties.
3. Select the General tab.
4. Toggle the Startup type to Automatic.
5. Press Apply.
6. Press the Start button.
7. Press OK.
8. Exit the Services dialog.