>>Since I have upgraded to Office 2007, whenever I try to open a document XLS that I have generated from .NET, I keep receiving this message:
>>
>>The file you are trying to open, 'Something.xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?
>>
>>If I answer yes, the file opens ok.
>>
>>Do I need to update my .NET environment to 2008 and recompile in order to have a XLS file generated by .NET to be ok once opened in Excel?
>
>Apparently a .XLS generated by Excel 2007 is not quite the same as one generated by Excel 2003:
>
>http://support.microsoft.com/kb/954318

Note that Microsoft doesn't admit that that is a bug in Office2007. Somehow it will be a problem in the consuming application.