Third-party applications can install extensions into Explorer, and sometimes those extensions are badly debugged and can cause Explorer to bomb at unexpected moments. For instance, Adobe Photoshop 7.0 installed an extension to allow Photoshop file thumbnails to show up in Explorer, but on my system, it sometimes caused Explorer to crashe. So I unregistered it.
If you want to see all of the available shell extensions that are currently installed, I highly recommend a freeware tool called ShellExView. Run it and you'll see everything that has installed some kind of hook into Explorer. If you see something that doesn't look right, or that was placed there by a program that was uninstalled or is now damaged, you'll be able to take care of the problem.
This was first published in December 2006