Documents saved as .sdw files were originally saved in old StarOffice or in early version of it's successor OpenOffice.org. Most open source word processors should still support this format and open it, however exporting it to an Excel spreadsheet (.xls, .xlsx) directly may be a little problematic.
Your best course of action would be to open the .sdw file in any word processor that allows it and manually copy and paste the bits you want to Excel so you can save it as .xlsx file.