To use PNG-24 in IE 5.5+ try:
<img alt="Transparant PNG" src="myimage.gif" style="width: 200px; height: 200px; filter:progid:
DXImageTransform.Microsoft.AlphaImageLoader(src='myimage.png', sizingMethod='scale')" />
there is more info available at http://people.brandeis.edu/~peelle/png/