From KB 831464:

n Microsoft Windows Server 2003 running Microsoft Internet Information Services (IIS) 6.0, static files that are compressed using gzip may become corrupted and may include content from other files on the Web server. If this behavior occurs, the page that is returned to the client is not rendered correctly. An access violation may also occur.

Translation: if you turn on Gzip compression for use with OWA 2003, your IIS server may get hosed. This patch fixes the problem.