How to Call a method via AJAX without causing the page to render at all?

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Response.Cache.SetCacheability(HttpCacheability.NoCache)
        If Request.HttpMethod = "GET" Then
            'do some work and return the rendered html
        ElseIf Request.HttpMethod = "POST" Then
            'do some work and return xml
            Response.ContentType = "text/xml"
            Response.Write("<data></data>")
            Response.End()
        Else
            Response.StatusCode = 404
            Response.End()
        End If
    End Sub