What is a virus (and what are trojans and worms)?
A (computer) virus is a program (a block of executable code) which attaches itself to, overwrites or otherwise replaces another program in order to reproduce itself without the knowledge of the PC user. Most viruses are comparatively harmless, and may be present for years with no noticeable effect: some, however, may cause random damage to data files (sometimes insidiously, over a long period) or attempt to destroy files and disks. Others cause unintended damage. Even benign viruses (apparently non-destructive viruses) cause significant damage by occupying disk space and/or main memory, by using up CPU processing time, and by the time and expense wasted in detecting and removing them. A Trojan Horse is a program intended to perform some covert and usually malicious act which the victim did not expect or want. It differs from a destructive virus in that it doesn’t reproduce, (though this distinction is by no means universally accepted). A dropper is a program which installs a virus or T
