Critical Update for Internet Explorer - MS04-040
http://www.microsoft.com/technet/security/bulletin/MS04-040.mspx contains information on a Cumulative Security Update for Internet Explorer. If you are running Internet Explorer, you should apply this update as soon as possible. Please note that Windows XP SP 2 and Windows Server 2003 are not affected. MS Knowledge Base article KB889293 describes issues that you may encounter with this update.
In addition to applying the update, please continue to use best practices including using the lowest-privilege account possible for your day-to-day use, to minimize the impact of issues such as this.