Yesterday, Microsoft released a critical security update for Windows intended to close a security vulnerability that could allow remote code execution. This update was released between Microsoft's regular updates because of it's seriousness in nature. According to Microsoft Security Bulletin MS08-067, it has been rated criticial for Windows 2000, Windows XP, Windows Server 2003, and important to all other supported versions of Windows. Go to the Windows Updates page to download yours now, or click on the Tools menu in Internet Explorer and select Windows Update.