<p>I have about 200 Excel files that are in standard Excel 2003 format. </p>

<p>I need them all to be saved as Excel xml - basically the same as opening each file and choosing <strong>Save As...</strong> and then choosing <strong>Save as type:</strong> <em>XML Spreadsheet</em></p>

<p>Would you know any simple way of automating that task?</p>

You could adapt the code I posted here:
http://www.atalasoft.com/cs/blogs/loufranco/archive/2008/04/01/loading-office-documents-in-net.aspx
It shows how to save as PDF (Word is shown in the blog, but if you download the solution, it has code for Excel and PPT).
You need to find the function for saving as the new format instead of exporting (probably the easiest way is to record a macro of yourself doing it in Excel and then looking at the code).