How to make my file a .htm?

Giganews Newsgroups
Subject: How to make my file a .htm?
Posted by:  David B (tall_walk…
Date: Thu, 24 Apr 2008

Using Win XP SP2

I wrote up a XHTML file using notepad. Now I want my browser to open
it as a web page but it only sees it as a .txt file. I tried changing
the extension to .htm but its still a .txt file. What do I do?