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Saving Excel worksheet in VB.NET

When saving an excel file in VB.NET
objExcel.SaveAs([FileName], fileformat:=[Number from below])

Find the format you want on the left or right, the number next to that format is your fileformat.

xlOpenDocumentSpreadsheet;60; OpenDocument Spreadsheet
xlAddIn ;18; Microsoft Office Excel Add-In.
xlCSV ;6; Comma separated value.
xlCSVMac ;22; Comma separated value.
xlCSVMSDOS ;24; Comma separated value.
xlCSVWindows ;23; Comma separated value.
xlDBF2 ;7; Dbase 2 format.
xlDBF3 ;8; Dbase 3 format.
xlDBF4 ;11; Dbase 4 format.
xlDIF ;9; Data Interchange format.
xlExcel2 ;16; Excel version 2.0.
xlExcel2FarEast ;27; Excel version 2.0 far east.
xlExcel3 ;29; Excel version 3.0.
xlExcel4 ;33; Excel version 4.0.
xlExcel5 ;39; Excel version 5.0.
xlExcel7 ;39; Excel 95.
xlExcel9795 ;43; Excel version 95 and 97.
xlExcel4Workbook ;35; Excel version 4.0. Workbook format.
xlIntlAddIn ;26; Microsoft Office Excel Add-In international format.
xlIntlMacro ;25; Deprecated format.
xlWorkbookNormal ;-4143; Excel workbook format.
xlSYLK ;2; Symbolic link format.
xlTemplate ;17; Excel template format.
xlCurrentPlatformText ;-4158; Specifies a type of text format
xlTextMac ;19; Specifies a type of text format.
xlTextMSDOS ;21; Specifies a type of text format.
xlTextPrinter ;36; Specifies a type of text format.
xlTextWindows ;20; Specifies a type of text format.
xlWJ2WD1 ;14; Deprecated format.
xlWK1 ;5; Lotus 1-2-3 format.
xlWK1ALL ;31; Lotus 1-2-3 format.
xlWK1FMT ;30; Lotus 1-2-3 format.
xlWK3 ;15; Lotus 1-2-3 format.
xlWK4 ;38; Lotus 1-2-3 format.
xlWK3FM3 ;32; Lotus 1-2-3 format.
xlWKS ;4; Lotus 1-2-3 format.
xlWorks2FarEast ;28; Microsoft Works 2.0 format
xlWQ1 ;34; Quattro Pro format.
xlWJ3 ;40; Deprecated format.
xlWJ3FJ3 ;41; Deprecated format.
xlUnicodeText ;42; Specifies a type of text format.
xlHtml ;44; Web page format.
xlWebArchive ;45; MHT format.
xlXMLSpreadsheet ;46; Excel Spreadsheet format.
xlExcel12 ;50; Excel12
xlOpenXMLWorkbook ;51; Open XML Workbook
xlOpenXMLWorkbookMacroEnabled ;52; Open XML Workbook Macro Enabled
xlOpenXMLTemplateMacroEnabled ;53; Open XML Template Macro Enabled
xlTemplate8 ;17; Template 8
xlOpenXMLTemplate ;54; Open XML Template
xlAddIn8 ;18; Microsoft Excel 97-2003 Add-In
xlOpenXMLAddIn ;55; Open XML Add-In
xlExcel8 ;56; Excel8
xlWorkbookDefault ;51; Workbook default 
example:
below will save your excel file in Excel version 95 and 97 format
objExcel.SaveAs("C:|NewExcelFile.xls", fileformat:=43)

Link to article:
http://msdn.microsoft.com/en-us/library/microsoft.office.interop.excel.xlfileformat.aspx