Unlike most other boot sector viruses, Form infects the DOS boot sector on hard drives instead of the Master Boot Record.
Form is only able to infect a hard disk when you try to boot the machine from an infected diskette. At this time Form infects boot sector, and after that it will go resident to high DOS memory during every boot-up from the hard disk. Once Form gets resident to memory, it will infect practicly all non-writeprotected diskettes used in the machine. Form will create bad sectors on disks it infects.
Form activates on the 18th of any month; on that day it will cause a 'click' from the PC speaker every time a key is pressed. On most machines this activation routine will not be heard, because the routine will fail if a keyboard driver (typically keyb.com) is loaded.
Form is one of the most widespread viruses in existance.
Note: If you have Form on a NTFS partition under NT, you need to repair the boot sector with a separate utility. A free program called BOOTPART can do this easily with this command:
BOOTPART WINNT BOOT:C:
BOOTPART can be downloaded from ftp://ftp.F-Secure.com/misc/anti-vir/bootpa20.zip
This is a pretty common minor variant of Form with no changes in the functionality.