CERT has other advice for avoiding Trojan horses . Viruses, Trojan horses and worms Different news outlets are variously describing ExploreZip as a Trojan horse, a worm, or a virus. Newer malware such as the Melissa Word macro virus is blurring the traditional boundaries between these three traditional classes. Computer viruses, like biological viruses, can't reproduce on their own. They must infect a file.
Viruses can infect and reproduce without user intervention if an infected program is run.
Viruses can't exist outside of the file they infect. ExploreZip doesn't require a file to exist, so it is clearly not a virus. Trojan horses must be run by the user in order to perform malicious tasks. Trojans exist on their own outside of other files. Because Trojans can't reproduce, they quickly die out.