Refuse to run unknown programs. The saying "no such thing as a free lunch" often applies, if something seems too good to be true like free latest release videos, free software it often comes with some extra code that you do not want. Make sure you are confident the offering is above board before you install it.
One of the fastest ways intruders can break security measures on hundreds of people's accounts is to take a seemingly innocent program and modify it to do something to registry or account files that makes a hole in the security measures.
Anti-virus software is extremely important to have installed and updated on your system at all times. There are hundreds of thousands of viruses that have been discovered and can be quite common in both business and academic environments. Anti-virus software can catch many stray diskettes infected with boot viruses and MS Word Macro viruses hidden in .doc files you may not expect.
But for all the viruses that have been identified, hun dr eds more are created and spread every day, many of which act more quickly than any anti-virus company can react to. For this reason it is especially important not to expect anti-virus software to save you from every malicious program you might run. If you stay determined about not exposing your account or system to unknown code and untrustworthy people, you will most likely not ever have to go through the devastating effects of a particularly damaging virus.