programatically password protect the excel sheet from the disk

I have an existing Excel file. I need to programatically password protect the file so that it can't be read unless the user provide the password.

I donot want to use INTEROP object like this

Dim gXL As Excel.Application, gWB As Excel._Workbook, gSheet As Excel._Worksheet ', gRng As Excel.Range ', debug As Boolean = False
    Const password As String = "Det gætter I aldrig!"

            gXL = New Excel.Application()
            Dim templateNAME As String = Server.MapPath(IIf(read_only, "template_readonly.xls", "template.xls"))
            gWB = gXL.Workbooks.Open(templateNAME)
            gSheet = gWB.ActiveSheet

            Dim rest As String = bog(gnames_width) + CStr(gMins.Count + nbr_of_headlines)
           gWB.Sheets(2).activate()
            gSheet = gWB.ActiveSheet
            gSheet.Range("A3", rest).Value2 = gnames
 If password <> "" Then gSheet.Protect(password, AllowSorting:=True, AllowFiltering:=True, AllowFormattingColumns:=True).

Please i donot want to use Excel.Application object or interop object.

Suppose my excel sheet is present at C:\temp\cost.xls.

I am generated the excel sheet using XSLT/XML .

Please provide any other way to protect my excel sheet.

Many Thanks.