If you’re trying to open an Excel 2003 XML file (SpreadsheetML) in Excel 2010 and getting “The file is corrupt and cannot be opened” error, try opening that file in WordPad (it will open as text XML) and save it back. Now try opening it in Excel 2010 again. You will still get “The file you’re trying to open is in a different format than specified by the file extension” warning, but after that the file should open.