>>>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
>
>Excel and Word files have such different formats in Office 2007 that they now have different extensions -- .xlsx and .docx. (Ken Levy must have been involved somehow <g>). I'm sure you knew that, was just pointing it out for others who may be using Office 2007.

That's true, but not what I was referring to. Excel 2007 can create "backwards-compatible" .XLS files (not .XLSX). According to the KB article, these .XLS files are not exactly the same as those generated by Excel 2003.