This indicates an attack attempt against a Buffer Overflow vulnerability in Microsoft Windows Media Encoder.
The vulnerability is in the ActiveX control "WMEX.DLL". It is caused by the application's inability to properly check the bounds of user supplied input. Successful exploitation may result in remote code execution.
Microsoft Windows Media Encoder version 9
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the following web site: