Trojan horses are programs that pretend to be legitimate software, but actually carry out hidden, harmful functions.

A Trojan program claims to have one function (and may even appear to carry it out), but actually does something different, usually without your knowledge. For example, DLoader-L arrives in an email attachment and claims to be an urgent update from Microsoft for Windows XP. If you run it, it downloads a program that uses your computer to connect to certain websites, in an attempt to overload them (this is called a “denial-of-service” attack).

Trojans cannot spread as fast as viruses because they do not make copies of themselves. However, they now often work hand-in-hand with viruses. Viruses may download Trojans that record keystrokes or steal information – and some Trojans are used as a means of infecting a computer with a virus.