I got this working nicely in VB.NET with a few simple alterations:

        Response.ContentType = "application/vnd.ms-excel"
        Response.AddHeader("content-disposition", "attachment;filename=Sample2.xls")
        Response.Charset = ""
        EnableViewState = False

        <!--Read in files and do replace here -->

        Response.Write(rptxml)
        Response.End()

Also a good tool for converting C# to VB is Instant VB from http://www.tangiblesoftwaresolutions.com/