Earlier versions of Microsoft Internet Explorer(IE6 and below) didn’t know how to handle the transparency in PNG image files.
Luckily this can (somewhat) be corrected using some Javascript.

Include the following function in either your site’s javascript file or in the head of your HTML document.

function bgPng(){
    for(i=0; i<document.all.length; i++) {
        var bg = document.all[i].currentStyle.backgroundImage;
        if(bg.match(/\.png/i) != null) {
            var mypng = bg.substring(5,bg.length2);
            document.all[i].style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(src=’"+mypng+"’, sizingMethod=’scale’)";
            document.all[i].style.backgroundImage = "url(‘/i/images/spacer.gif’)";

You will now need to have the following code execute everytime the page has successfully loaded. This will basically convert everything on the current page from background images to AphaImageLoaders via the Microsoft specific CSS filter.

if (navigator.platform == "Win32" && navigator.appName == "Microsoft Internet Explorer" && window.attachEvent) {
    window.attachEvent("onload", bgPng);