A computer virus can have many effects, such as deleting or corrupting files, replicating itself, affecting how programs operate or moving files. Some common types of viruses include resident viruses, overwrite viruses, file infectors, directory viruses and boot viruses. A computer virus is a type of malware that is designed to enter a user's computer without permission.
Resident viruses are permanent infections that exist in a computer's RAM memory. These viruses are capable of interrupting actions that the operating system attempts to execute. They can also corrupt files and programs that are opened, closed, renamed or copied.
Overwrite viruses infect files and then delete the information stored in the infected areas, rendering the files unusable. Data infected by overwrite viruses is difficult to retrieve, and the virus can typically only be removed by deleting infected files completely.
File infectors infiltrate executable programs or files, activating themselves only when the program is run. File infectors account for a large portion of existing viruses.
Directory viruses alter directory paths, changing the location of infected files. When the infected programs run, they operate from a new location determined by the virus. This makes directory viruses difficult to locate and neutralize.
A boot virus infects a computer's hard drive, making it unable to boot up. In earlier eras of computing, boot viruses could also be found on infected floppy disks.