A Worm is.....

A type of virus that replicates itself over a computer network. This almost always does something malicious, such as consuming computer resources and freezing up processes. Similar to a virus, where infected computers spread the problem to all users associated with the computer (via email or internal network).

Software such as Platinum Antivirus prevent virii and worms and also help to clean infected files if they are suspect to these malicious programs. Usually this will get on a computer without the user's knowledge. The user will not find out generally until the program has run its course and done whatever bad things it had intended to do.