1. Open Windows Live Messenger (Start → All Programs → Windows Live Messenger).
2. Click Tools → Options and then click File Transfer from the menu on the left.

3. Select the Scan files for viruses using option. Directly below this option, add the directory path to the ecls.exe file. At the end of the path, you can also add logging and cleaning parameters, if desired. The path should look like this:

   “C:Program FilesESETESET NOD32 Antivirusecls.exe” –log-file=log.txt –action=clean

   NOTE: If you are using ESET Smart Security the correct directory path is “C:Program FilesESETESET Smart Securityecls.exe” –logfile=log.txt –action=clean
   

   The “–log-file=log.txt” parameter will create a scan log in C:Program FilesESETESET NOD32Antiviruslog.txt of all files received through Windows Live Messenger. The “–action=clean” parameter will automatically attempt to clean any infected files.

   For additional information on available parameters using the command line scanner, please reference the help files included with the program or section 5.3, “Command Line” of the ESET NOD32 Antivirus 3.0 manual.

source http://www.eset.com.sg/html/167/682/