Created WEB-INFO folder. Tried to create new file within that folder, with name web.xml, but got immediate error dialog that says: Cannot open default editor on web.xml. Unable to instantiate editor: org.jboss.ide.eclipse.jdt.j2ee.xml.ui.editors.WebXmlEditor java.lang.NoClassDefFoundError:
com/sun/org/apache/xerces/internal/jaxp/SAXParserFactoryImpl

Quote:

org.jboss.ide.eclipse.jdt.j2ee.xml.ui.editors.WebXmlEditor

We don't ship this class. Its provided by another product you have installed so you should consult them for a workaround. If you'll install MyEclipse on a clean Eclipse 3.0.2 installation you shouldn't have any further problems.

I started over with a fresh Eclipse 3.0.2 installation. Then I purchased, downloaded, and installed MyEclipse. I still get the same error.

I should mention that this only happens when I first create a .xml file, or when I double-click on an existing .xml file. If I right click and select Open with <some editor> it works fine.

As Scott mentioned we don't ship that class, so what is likely happening is that you are reusing the same workspace which stores plugin cache data as well as file association information, so I'm guessing in your old workspace your XML files were associated with the JBossIDE's XML editor and not the MyEclispe XML Editor. You can fix this by navigating to Window > Preferences > Workbench > File Associations and changing the .xml association to use the "MyEclipse XML Editor" by default.