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. Worms are similar to Trojan horses in that the worm file can exist on its own, and often masquerades as a useful file. The difference is that worms can reproduce and spread, whereas Trojans almost always die out quickly. ExploreZip's ability to copy itself through email qualifies it for classification as a worm. Previo u s week 56K.COM is written and maintained by Les Jones.