The bug affects all of the current versions of Windows, from XP up through Windows 7 and Windows Server 2008. Microsoft issued an advisory about the MHTML vulnerability, which has been discussed among security researchers in recent days. The vulnerability exists due to the way MHTML interprets MIME-formatted requests for content within a document. Microsoft is aware of published information and proof-of-concept code that attempts to exploit this vulnerability. The side effects from enabling the FixIt workaround are minor, Microsoft officials said.
Source: https://threatpost.com/microsoft-warns-mhtml-bug-windows-012811/74892/