There is no such thing as just "XML format". XML is a kind of a media for many formats. This way, the question does not have a full sense. I just want to bring to your attention that an .XLSX Excel document is already XML; you only need to unpack the file, which is actually is a ZIP format. You can learn about the format here:
http://en.wikipedia.org/wiki/Office_Open_XML_file_formats[
^],
http://en.wikipedia.org/wiki/Office_Open_XML[
^].
What you are going to do with that is your business. From your question, I'm not sure you really know what.
[EDIT]
For working with ZIP, you can use #ziplib:
http://www.icsharpcode.net/opensource/sharpziplib/[
^].
Another option is using SevenZipSharp, a .NET wrapper of the famous 7-Zip:
http://en.wikipedia.org/wiki/7-Zip[
^],
http://sevenzipsharp.codeplex.com/[
^].
Both ZIP libraries are open-source.
Good luck,
—SA