请问怎么用javascript打开一个保存文件的对话框

<HTML>
<HEAD>
<title>WebForm1</title>
<meta name="GENERATOR" Content="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" Content="C#">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
<script language="javascript">
    
function SaveAs()
{
event.returnValue=false;
show.window.location.href="C:\Documents and Settings\Administrator\桌面\新建 文本文档.htm";
timer=setInterval(CheckLoad,100);
}
function CheckLoad()
{
if(show.readyState!="complete")
{
show.document.execCommand("SaveAs");
clearInterval(timer);
}
}
</script>
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<INPUT onclick="SaveAs()" style="Z-INDEX: 101; LEFT: 8px; POSITION: absolute; TOP: 8px" type="button" value="Button">
<iframe style="BORDER-TOP-WIDTH: 0px; BORDER-LEFT-WIDTH: 0px; BORDER-BOTTOM-WIDTH: 0px; WIDTH: 0px; HEIGHT: 0px; BORDER-RIGHT-WIDTH: 0px"
name="show"></iframe>
</form>
</body>
</HTML>