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You can try XMLNotepad, which is developed by microsoft. You can get many tools, if you do a search on google. Now many editors are having xml facility as add ons. XmlTidy is used with TextPad.
XML Notepad is a very useful tool as it makes it dificult to screw up your XML validation. The only issue with it is you have to be carefull when inserting values as the left side does not quite line up with the right side. I have made a couple of mistakes in the past. (When you download it and look at XML what I am saying will make sense.)On another note,I find it very easy if I select expand all this way I am looking at the entire XML message.