Show your File Extensions!
Have you recently upgraded to Microsoft Office 2007 or 2010? By turning on your file extensions in Windows Explorer, you can see if you are opening an older version or the newest version of a Word document or an Excel spreadsheet. All of your old files end in a .doc or .xls extension and all of your new documents will end in a .docx or .xlsx extension (or .docm/.xlsm if containing macros).
Tools > Folder Options > View tab. Ensure the check box for Hide extensions for known file types is cleared. In Windows 7 or Vista, click Organize in Windows Explorer > Folder and Search Options > View tab to access the same option.