A security problem has been recognized that could permit an unauthenticated remote attacker to compromise your machine and gain control over it. You can help protect your machine by installing this update from Microsoft. After you install this update, you may have to reboot your computer.
Vulnerability in the Windows Kernel that could allow remote code execution.

Executive Summary

this security update fixes some privately reported vulnerability in the Windows kernel. The mainly serious vulnerability could permit remote code run if a user viewed a specially build EMF or WMF image file from an infected machine.

This security update is rated Critical for all supported editions of Microsoft Windows 2000, Windows XP, Windows Server 2003, Windows Vista, and Windows Server 2008.

In particular for Windows Vista Service Pack 1 users:

Windows Vista Business
Windows Vista Enterprise
Windows Vista Home Basic
Windows Vista Home Premium
Windows Vista Starter
Windows Vista Ultimate
Windows Vista Enterprise 64-bit Edition
Windows Vista Home Basic 64-bit Edition
Windows Vista Home Premium 64-bit Edition
Windows Vista Ultimate 64-bit Edition
Windows Vista Business 64-bit Edition