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 Both Viruses and Trojans are directly related to your Hard Drive .
A Hard Drive is a media storage device which is built into the computer tower and serves as a storage space for programs and other data.
A Hard Drive
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 You can think of your hard drive as a container that holds your operating system, program files and all other data that you store on your computer.
Empty Hard Drive
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This storage container is subject to be infected by viruses in the same way it is subject to store music or pictures. A hard drive can not prevent you from storing data and it can not stop viruses from being stored in it.
Hard Drive containing data
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A virus is a program, on your hard drive, that runs like any other program but to its own benefit and not to your benefit.
 
 
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In the past viruses used to be simple programs that were removed using anti-virus tools
 
Virus being removed
 
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Virus-free Hard Drive
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A virus infecting a Hard Drive
 
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Viruses today are complex programs that can outwit the smartest operating system. Their behavior is similar to a drop of ink into a container full of water.
 
Infected Hard Drive
 
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They can also outsmart anti virus tools by "hiding" in restricted areas such as System Restore or registry, system files or Java scripts and thus are not easily detected by anti virus programs or Registry editors
 
Anti virus programs may not always work
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Some viruses may remain on your hard drive even after anti-vius programs remove obvious viruses 
 
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The guaranteed antivirus removal method is by restoring the hard drive to factory default  
 
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then install the operating system and program files. 
 
         
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