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If the file is well-formed or valid XML, you can just open it with any XML-conformant browser (see What do I have to do to use XML? and Where can I get an XML browser?). This will display the file in an unformatted view, showing all the markup in a format that lets you fold up or unfold the nested hierarchy (click on the little plus and minus symbols), which will at least let you read something.
If the file contains a link to an XSLT or CSS stylesheet (and the stylesheet was provided or is web-accessible) then the browser should format the file in a readable manner (but beware that in-browser formatting is not robust, and only supports a subset of XSLT 1.0).
If you want to edit the file, you need an XML editor (see XML Editors). Unless you are very skilled with pointy-bracket markup, do not try to edit XML files with non-XML editors.