All versions of Internet Explorer prior to 7 cannot correctly display the transparent PNG-24 images. In place of the transparent pixels are light gray pixels. The solution that I like the most is called the IE PNG Alpha Fix. It uses a .htc file that corrects the behavior of IE using javascript. The fix also requires the a CSS hook:

* { behavior: url(iepngfix.htc); }

I have used this solution on a couple of projects now, it works well and doesn’t require gratuitous HTML hackery.