From the Excel Help file:
Control the startup prompt for updating all links
Don't ask whether to update links when I open any workbook, and update links automatically
This option is for the current user only, and affects every workbook opened. Other users of the workbook are not affected. This option also affects links to other programs.
On the Tools menu, click Options, and then click the Edit tab.
Clear the Ask to update automatic links check box. If the box is cleared, the links are automatically updated, and no prompt is displayed.
Don't ask to update links for this workbook, and let me control whether links are updated
Warning This option affects all users of the workbook. If you choose to not update links, and not to prompt, users of the workbook will not know the data is out of date.
On Edit menu, click Links.
Click Startup Prompt.
Select the option you want.
Note You will still be notified if there are any broken links.