How do I prevent my exported xls files from causing a security message in Excel 2007

Using different techniques to export an xls file on the web using c# asp.net, I receive the following security message when opening with Excel 2007:

"The file you are trying to open, <filename>, is in a different format than specified by the file extension..."

Though it will open if you hit ok.  Yet Excel 2003 will open it without the security alert.  The two techniques I've tried is htmlstreaming and tabbed delimited.  If I use xml formating will that solve the problem?  This forum says no: http://forums.asp.net/p/1070490/1560113.aspx.  But even if xml formating solved the problem, will it work for Excel 2003 or Excel 2000?

Thanks