XP Theme partially changed on boot, and screensaver also changed.

When I switched on the computer today, the main area of the desktop was in Windows Classic theme, which is what I chose. However, the task bar at the bottom was in Windows XP/Fisher Price/Teletubbies colours - quite ghastly.

Additionally, trhe screensaver was also changed to Windows XP.

I have seen many reports on the internet of theme changes, completely rather than partially, and none of those reports mentioned an associated screensaver change.

What might have caused this?