Originally Posted by Kaneto

This sounds like a case of Excel's Extension Hardening, which was introduced in version 2007. Does the error message say that the file is in a different format than specified by the extension?

Some more info on Extension Hardening and how to disable it can be found here:
http://blogs.msdn.com/b/vsofficedeve...n-warning.aspx

Hope this helps.

-SW