This may not be the answer you were looking for, but Excel already has built-in support for XML. If you are using Excel 2007, you will need to enable the Developer tab on the ribbon. From there you can import and export XML schemas and files.
To enable the developer tab:
Click the Microsoft button...

Click Excel Options
- Check "Show Developer Tab in the Ribbon"
You will need to create a map by either importing an example XML file that is in the same format as the one you'd like to export, or you can click Source and use a schema (XSD) file.
From there, you just need to map the elements in your XML Map to the data in your sheet, then export the data to an XML file.