My reasoning behind this is that nine times out of ten your antivirus software will recognize the infected file when you attempt to copy it. Most virii propagate by using addresses they have found on a victims contact list. In summary, always check attachments and verify that they are clean before opening. Last but certainly not least is physical security of your machines. Keep them locked up as much as is practical.
Lets face it, anyone with a bootdisk can have control of an unlocked computer.
Machines with sensitive data should always remain under lock and key. Many hackers can exist inside of your network.