Question: I have xml configs which are very complex. They are validated using dtds. I am looking for some application which reads the dtds and provides a GUI interface to write xmls. So that we don't have to write xmls by hand. If there is nothing existing already, how to start with developing one?

Answer: 
did you try [Eclipse](http://www.eclipse.org/)?
You can edit XML files according to the DTD.

If your audience is not development oriented, you can have a look to [Editix XML Editor](http://www.editix.com/) (the free version is already fully usable) or [Notepad++](http://notepad-plus-plus.org/) (a strange mix between the classic windows notepad and eclipse ;) ).

Otherwise, if you want to write come code, have a look to [wxPython](http://www.wxpython.org/) there are some widgets to edit/view HTML and XML.
Take a look at [oXygen XML Author](http://www.oxygenxml.com/xml_author.html). It's not intended for programmers/developers like oXygen XML Editor is. It's a WYSIWYG XML authoring tool. You can create CSS stylesheets to display the XML in a way that is easy for the authors to create/modify the XML.

It's not free and you'll have to create CSS for display, but it is *much* better than trying to develop something from scratch. It also supports creating templates so that authors can start with a base XML file and then modify/update.