What version of IE? Ah, mass email.
IE6 simply doesn't support normal PNG transparency without running the AlphaImageLoader script mentioned in the page you linked to. That's one of the many reasons IE6 sucks. There's no way around this, I'm afraid.
IE7, on the other hand, should handle embedded PNG images with alpha just fine without any special code.
You could use this script in your code:
http://homepage.ntlworld.com/bobosola/ This only runs if the browser is IE 5.5 or 6.