Types of Damages and malfunction a virus may do:
- Format hard disks.
- Encrypt hard disk data
- Corrupt file allocation table
- Corrupt directory
- Corrupt files
- Change the screen display
- Display some messages
- Make the system behavior funny.
- Reset/Hang the system.
- Lock keyboard.
- Interfere with printing
- Lock the floppy drive
- Corrupt floppies
- Damage the BIOS
- Alter your documents (Macro Viruses)
- Change the Calculations and Formulas (Most of the Excel Macro Viruses)
- Delete Files
and so on...