Trojans;
Which are a destructive program. A computer trojan horse is a program which appears to be something good, but actually conceals something bad. Data commonly targeted by trojan horses includes usernames and passwords, but a sophisticated trojan horse could also be programmed to look for items such as credit card numbers. A more common method of spreading a trojan horse is to send it via e-mail. The attacker will send the victim an e-mail with an attachment called something like "prettygirls.exe." When the victim opens the attachment to see the pretty girls, the trojan horse will infect his system. A similar technique for spreading trojan horses is to send files to unsuspecting users over chat systems like IRC, AIM, ICQ, MSN, or Yahoo Messenger.
Worms;
Which are classed as a sub form of virus but with the ability to self replicate its self and shut down your computer, a computer worm can run itself. A computer worm can spread without a host program, although some modern computer worms also use files to hide inside. In addition to replication, a worm may be designed to do any number of things, such as delete files on a host system or send documents via email. A common task is for a worm to install a backdoor in the infected computer threatening Windows and Dos programs.
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