Windows Explorer has stopped working

Error message:
Windows Explorer has encountered a problem and needs to close. We are sorry for the inconvenience.

As a result I cannot search anything on my PC.

Then when the window closes it always takes the desktop out with it. This seems unusual.
Microsoft Windows has experienced an expected error. As a precaution your active desktop has been turned off.

The desktop is easily repaired and, apart from Windows Installer (see different thread), the PC works very fast and efficiently. I can open folders and files manually. Only the Microsoft software is playing up!

Windows XP Media Center Edition, Version 2002, SP3.

Any ideas, please?

Thanx Jon