After a search for using XSLT to produce Microsoft Excel spreadsheets I found this excellent blog by Brian Jones a program manager in Office. I was looking for solutions to a bug one of our customers encountered with using calculations in the Excel files. I thought there was a difference in the number format as he was using a Norwegian version of Excel, but turns out it was Excel 2000 and you need to have Excel XP or 2003 with Excel files created with SpreadsheetML.
SpreadsheetML is a powerful way of creating Excel spreadsheets. So if you have some data that you have exported to XML you can transform it to Excel by using the XSLT. See Brians blog for a good introduction to how to achieve it.