Okay, that was what I figured out too, and now it is confirmed.
Coming to Virusscanners I have so far not had anything infected coming in. I use McAfee v8.0.41 and I haven't found in the documentation that it specifically scans Messenger but I think it really is scanning all incoming files. The new McAfee v9.0.10 says that it scans MSN Messenger and other messengers too. I guess I have to wait and see or ask somebody to send me an infected file - NOT.
Saw You were in here Patchou.
Many Thanks for a very, very nice Add-on to Messenger. You've done a great job. Keep it going