When should have NS caught these viruses?


When should have NS caught these viruses?

Last week I moved from one hosting company to another.  I zipped my mail file from the old company, copied it to my PC, and then moved it to the new company. 

The zipped email file contained viruses in the SPAM folder.   

These were not detected when I downloaded the file to my machine.

Today, on my weekly full system scan, NS found viruses in the zipped email file.

My questions:
Should NS have detected the viruses in the zipped email file when I downloaded it to my PC?

Why did I have to wait for it to be detected in the full system scan?

I would prefer Norton to catch these things as soon as possible.