Below are examples of using IE's CSS Behaviors to fix border radius and png transparencies.
http://www.htmlremix.com/css/curved-corner-border-radius-cross-browser
http://scarfoo.com/archives/16
http://www.twinhelix.com/css/iepngfix/
http://msdn.microsoft.com/en-us/library/ms532147(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/ms531078(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/ms531079(v=vs.85).aspx