Why trojans are often called viruses…
A virus is a programm that adds itself to a file and forms a modified .exe-file – that’s why you can’t recognize it as a virus file at first sight or remove from the file system. The most common charactaristic of a virus is that it is able to spread itself on its own impulse when it has been activated; further distribution occurs by copying infected files over networks – for example per email. Viruses can delete or change files/data, to delete them efficiently the boot sequence has to be eliminated out of the infected executable file.
The primary difference between virus and trojan is that trojans don’t spread on their own. A trojan is a standalone program that makes certain areas of interest for other users/computers on the internet available. Therefore it has to be run at every system reboot, what’s done by autorun keys in the system registry. A trojan is an .exe file that hides somewhere in the system but in the opposite to viruses it removable manually – if you know how to do it.
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