A Virus or a Trojan Horse?
Virus: A program or file that spreads itself by infecting other programs on the computer. Viruses can do serious damage, such as erasing your files or your whole disk. Viruses cannot spread to a new computer without your help such as an email attachment, or networked with other computers.
Trojan-Horse: Program that appears harmless, but actually contains harmful information. This harmful information could be a popup pretending to be a legitimate anti virus program that finds hundreds of bogus infected files and attempts to get you to pay for it. It could also contain a virus or worm, which could then spread the damage throughout the computer.