The default file format in which the files are saved in Excel 2010 is *.xslx. But if you are one of the very earlier user of Excel 2010 in your organization and rest of the users are still using the previous version of Office then they will have compatibility issues when working with your excel files. To avoid these compatibility problems you can change the default file save format to previous version i.e .xls format.
Click the File menu then the options link. In the Excel option screen, click on the Save options tab.
In the Save Options screen, navigate to Save workbooks section. Using the Save files in this format dropdown, you can set the file format to Excel 97-2003 Workbook (*.xls).
Also if you want o preserve the visual appearance of the workbook then you can choose the colours that will be seen in previous versions excel by clicking on Colors button.
After making the required changes click on the Save button to confirm and save the changes.