Information
Viruses - A virus is a small piece of software that piggybacks on real programs. For example, a virus might attach itself to a program such as a spreadsheet program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to reproduce (by attaching to other programs) or wreak havoc.
E-mail viruses - An e-mail virus travels as an attachment to e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book. Some e-mail viruses don't even require a double-click -- they launch when you view the infected message in the preview pane of your e-mail software.
Trojan horses - A Trojan horse is simply a computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your hard disk). Trojan horses have no way to replicate automatically.
Worms - A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.
How viruses may affect files?
Viruses can affect any files; however, usually attack .com, .exe, .sys, .bin, .pif or any data files - Viruses have the capability of infecting any file; however, will generally infect executable files or data files, such as word or excel documents that are opened frequently and allow the virus to try infecting other files more often.
Increase the files size - When infecting files, virtues will generally increase the size of the file; however, with more sophisticated viruses these changes can be hidden.
It can delete files as the file is run - Because most files are loaded into memory, once the program is in memory the virus can delete the file used to execute the virus.
It can corrupt files randomly - Some destructive viruses are not designed to destroy random data but instead randomly delete or corrupt files.
It can cause write protect errors when executing .exe files from a write protected disk - Viruses may need to write themselves to files that are executed; because of this, if a diskette is write protected, you may receive a write protection error.
It can convert .exe files to .com files - Viruses may use a separate file to run the program and rename the original file to another extension so the exe is run before the com.
It can reboot the computer when executed - Numerous computer viruses have been designed to cause a computer to reboot, freeze, or perform other tasks not normally exhibited by the computer.
What viruses may do to a computer:
Below are different issues you may experience when you are infected with a virus. Keep in mind that you also may be experiencing any of the below issues by another computer related issue and not a virus.
- Deleted files.
- Various messages in files or on programs.
- Changes volume label.
- Marks clusters as bad in the FAT.
- Randomly overwrites sectors on the hard disk.
- Replaces the MBR with own code.
- Create more than one partition.
- Attempts to access the hard disk drive, which can result in error messages such as "Invalid drive specification".
- Causes cross-linked files.
- Causes a "sector not found" error.
- Cause the system to run slow.
- Logical partitions created, partitions decrease in size.
- A directory may be displayed as garbage.
- Directory order may be modified so files, such as COM files, will start at the beginning of the directory.
- Cause Hardware problems such as keyboard keys not working, printer issues, modem issues etc.
- Disable ports such as LPT or COM ports.
- Caused keyboard keys to be remapped.
- Alter the system time / date.
- Cause system to hang or freeze randomly.
- Cause activity on HDD or FDD randomly.
- Increase file size.
- Increase or decrease memory size.
- Randomly change file or memory size.
- Extended boot times.
- Increase disk access times.
- Cause computer to make strange noises, make music, clicking noises or beeps.
- Display pictures randomly.
- Unusual or undocumented error messages.