However, if you frequently share word processor documents, spreadsheets or small applications with friends you probably have a habit of opening anything that comes from certain people. You are at risk for Melissa, PrettyPark and ExploreZip. These virus programs can address their malicious email as being from your friend and even greet you by name. Any slight variation in writing style or typical habits should alert you to check with your friend before opening the file. The caution is just because in spite of your diligence your anti-virus program could still miss a very new virus.
Healthy skepticism is always a lifesaver.
As for anti-virus programs, you should regularly upgrade your program and virus definitions. These definitions are identifiers that the scanner program uses to find malicious computer code. All leading anti-virus software companies post new definition files at their web site for free download at least every month. Quite often surprise virus releases require more frequent updates. First make sure you have an anti-virus program.