Security researchers are anticipating the release of programs that could take advantage of computers using Windows operating systems that have not received a critical security patch for the Messenger Service (MS03-043). If your computer has not been patched, a flaw in the Messenger Service could permit an unauthorized person to gain control of your computer. Please visit the Microsoft Windows Update Web site to verify that this patch has been applied. If missing, this patch and any other missing critical security patches should be applied as soon possible.
Refer to Microsoft Knowledgebase Article #828035