Hi,
You will need an InfoPath form template to open a non-InfoPath created XML. If you are looking to just edit an XML file, you can use any text editor, such as Notepad, or a free application such as XML Notepad (http://www.microsoft.com/downloads/details.aspx?familyid=72d6aa49-787d-4118-ba5f-4f30fe913628&displaylang=en).
- Dipesh