Scan result: No virus threat detected

Hey All,
Can antivirus programs really scan a File type: video/mpeg before it is downloaded? I am getting calls about a certian mpeg file that is going around in an emial which is supposed to be scanned by Norton and says
Scan result: No virus threat detected Yet when they open the file AVG says its a trojan. Can these scans be trusted ? Is it just a bad dream? It is Yahoo mail scanned with Norton Antivirus.
Thanks,
Dan

 

Look at Halo's post, maybe it has to do with that issue.

http://forums.majorgeeks.com/showthread.php?t=118958

 

main problem is norton's anti-virus. Nortons comes pre packeged with spyware and trojans.

 

My mail coming in from a yahoo acct. is scanned, maybe twice in yrs. of using it I have gotten a msg. similar to, Unable to scan download file, but there is a Big Warning if file was not scanned, I think it's in Red.

I rescan downloads with AVG, but I think it's in Yahoo's best interests to scan the mail, they would not want their members passing along viruses. To date I have not found or downloaded a virus from yahoo mail and I do download some videos from friends, but I'm pretty careful in what I download.

It could be a false positive or a new virus that is not in the def. file yet, the Trojan name was not mentioned, so can't look up.