When opening a file in Excel, why does it say the file is corrupted?

Microsoft added a new "security feature" to Excel that causes an error message to be displayed when you try to open an XLS file downloaded from the Event Planner application. This is not a bug on our part. Microsoft (again) broke the proper functioning of their own software.

Here is a full explanation of the issue.

http://blogs.msdn.com/vsofficedeveloper/pages/Excel-2007-Extension-Warning.aspx

The two suggested solutions:

1) Click OK when you open the file and receive the security prompt.
2) Edit your registry to disable the prompt