Originally posted by Paul Clapham: Is the MIME type of whatever you are complaining about actually "text/html"?
Pauls questions seems more relevant. MSIE often ignores the Content-Type heading and displays things as HTML whether that was the author's intent or not.
What technology are you using to generate the HTML (JSP, PHP, ASP...)? Are you specifically setting the content-type header? If you have the LiveHTTPHeaders plugin for FireFox installed, you can see what header is being used.