Change the default file format for saving workbooks in Excel 2007

By default, in Excel 2007 the workbooks are saved in .xlsx file format. You can change the default file format for saving workbooks by performing the following steps:
1. Click the Microsoft Office Button , and click Excel Options.
2. From the Excel Options dialog box, click Save from the left pane.
3. In the right pane, under Save workbooks, click the Save files in this format: dropdown box and select the file format that you want to use by default.
4. Click OK to close the Excel Options dialog box.