protected void OnRowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            Desc = (String)DataBinder.Eval(e.Row.DataItem, "Act_Response");
            Actid = (int)DataBinder.Eval(e.Row.DataItem, "Actid");
            if (Desc != null)
            {
                if (Desc.Length > 30)
                {
                    Desc = Desc.Substring(0, 30) + "...";
                    Desc = "<a href='../Activity/viewActivity.asp?ActID="+Actid+"' onclick='javascript:window.open(this.href,'scrollbars,width=650,height=600'); return false'>" + Desc + "</a>";                    
                   
                }
            }            
        }
    }

hi window.open is not working in Gridview .

and html is

<asp:TemplateField HeaderText="Description">
                                            <ItemTemplate>
                                                <%#Desc%>
                                            </ItemTemplate>
</asp:TemplateField>

Add a Label to your template instead  like:

           <asp:TemplateField HeaderText="Header 3">
                <ItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text="Click Me"></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>

Then at RowCreated event you can do something like this:

    protected void Gridview1_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            Label l = (Label)e.Row.FindControl("Label1");
            if (l != null)
            {
                string script = "window.open('Default.aspx');";
                l.Attributes.Add("onclick", script);
            }
        }
    }