Critical security vulnerability: Microsoft warns of worms - "Windows 2000, XP, and Windows Server 2003 are particularly affected by this error. Microsoft classifies the issue as critical here. Windows Vista and Windows Server 2008 are also affected, but Microsoft classifies the issue as important here because users must authenticate themselves first. Even users who are already using a pre-beta version of Windows 7 are advised to install the update." - so basically everyone.