Been getting this question on and off now for a couple of weeks, so I thought I would drop a line about it.

Excel Services ONLY supports two of Excels file formats - the new XLSX and XLSB ones. Other files will raise an error saying that the file is not supported (the actual error message says that it's either unsupported, corrupt or has IRM).

If you only have Excel 2003, we apparently have a compatibility pack that will let you save XLSX/XLSB files from it, allowing you to open these files in Excel Services even if you do not happen to have Excel 2007 client installed on your machine.