Most probably when installing the Anti-Virus you have not unloaded the mail program and the library which is necessary for the module functioning failed to be installed correctly.
1. Close Microsoft Outlook.
2. Go to Start > Run
3. in the open window enter
regsvr32 -u <"full_path_to_Anti-Virus_setup_folder>\mcou.dll".
For example, for the application Kaspersky Internet Security 6.0, installed to the setup folder the path will look the following way: regsvr32 -u "C:\Program Files\Kaspersky Lab\Kaspersky Internet Security 6.0\mcou.dll".
4. Click OK.
5. Open the same window and enter
Unlike the previous line the “-u” parameter is missing in this line.
6. Click OK.