Excel 2007 user warning when opening xls files

1) Exit Excel 2007.
2) Start Registry Editor.
3) Locate and then click the following registry subkey:

HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security

4) On the Edit menu, point to New, and then click DWORD Value.
5) Type ExtensionHardening, and then press ENTER.
6) Right-click ExtensionHardening, and then click Modify.
7) In the Value data box, type 0, and then click OK.

The following list contains the value data settings that are appropriate for the ExtensionHardening setting:

0: Do not check the file name extension and the file type, and bypass the function of the warning message.
1: Check the file name extension and the file type. If they do not match, display the warning message.
2: Check the file name extension and the file type. If they do not match, do not open the file.

Note: The default value data is 1. When the value data is set to 1, the behavior becomes the same as when no registry value is set. When the value data is set to 0, the file name extension and the file content are not checked in all situations. It is not recommended bypassing this function.

8) On the File menu, click Exit to exit Registry Editor.

You say the files are "shared files". Does that mean that you have the "Share Workbook" feature enabled? If so, don't worry any more. Sharing workbooks so that multiple users can edit a file is a recipe for disaster. Shared files will eventually become corrupt, impossible to troubleshoot or repair. It's not a question of if it will happen, but rather when it will happen.

There is nothing you can do to repair corrupt shared files.

If you need simultaneous multi-user read/write access to your data, then Excel shared files is NOT the way to go. This Excel feature is riddled with bugs and results in unexpected behaviour.

If you use shared files, be certain to have a strict backup regime, because the files can become corrupt at any time.

For safe multi-user edit access to your data, you may want to explore a database solution like Access or SQL instead. Excel can still be the front end for data entry and reporting, but it was never designed to be a multi-user interface for Excel data.