MHTMLRedir.Exploit

This threat contains specially-crafted, HTML code that can download and execute programs without prompting you. This threat only affects Microsoft Internet Explorer.  When visiting a Web page or receiving an HTML email that contains this threat, a file can be downloaded and executed. Under normal conditions, Internet Explorer would prompt you before allowing any executable content to be downloaded and executed on the system. This vulnerability in Internet Explorer allows specially crafted HTML to bypass this security prompt.

Microsoft has released patch MS04-013 (KB837009) to address this issue.

http://www.microsoft.com/technet/security/bulletin/ms04-013.mspx