Sorry if this is well off the target. I had exactly the same problem but when I changed to ZoneAlarm (from Norton) the new programme gave me the oportunity to use it to scan Messenger. Have you looked at Avast options to see if will do the same?
It seemed to confirm to me that the path I gave in my original post of 12 April at 20:53 is the correct path to use in MSN Messenger for the .exe file which will launch Avast to virus scan any files that my 2 teenage daughters may download when using MSN Messenger!!