Hi,
First, in IIS Manager, go to web site Properties, Home Directory, Configuration..., Mappings.
Map .htm extenstion to C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\aspnet_isapi.dll, (here I'm using asp.net 1.1)
Second, add httphandler in your web.config like this:
<httpHandlers>
<add path="*.htm" verb="*" type="System.Web.UI.PageHandlerFactory"
validate="true" />
</httpHandlers>
Third, create a sample .htm, e.g. sample.htm under your web application.
<%@ Page Language="C#" AutoEventWireup="true" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<% Response.Write("hello world"); %>
</div>
</form>
</body>
</html>
You can find it is actually parsed by asp.net.