The use of System.IO and regular expressions makes this a very easy task. Place a <asp:Literal> control (ID="htmlbody") on your page, and then use the following code to strip out everything up to and including the <body> tag (regardless of whether the tag contains additional attributes), and everything from the closing </body> tag onwards:
StreamReader sr;
string html;
sr = File.OpenText("<path_to_file.htm>");
html = sr.ReadToEnd();
sr.Close();
Regex start = new Regex(@"[\s\S]*<body[^<]*>", RegexOptions.IgnoreCase);
html = start.Replace(html,"");
Regex end = new Regex(@"</body[\s\S]*", RegexOptions.IgnoreCase);
html = end.Replace(html, "");
htmlbody.Text = html;
