Computer viruses are basically programs that are written and developed by destructive people. If they enter your system, then they will infect the file stored. Whenever you restart your computer or execute the infect file or program, the virus gets active. Until you do not use the disk where the infected file is stores, the virus is inactive. Once it is loaded and saved your computer and save in the disk of the machine, the computer will crash down it. Hence, it is wise to format the system. Otherwise, they might damage you data, delete files or erase the entire hard drive. Viruses are of different types; some display messages; create sounds or send verbal comments.