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What boot sector viruses do
Boot sector viruses infect or substitute their own code for either the DOS boot sector or the Master Boot Record (MBR) of a PC. The MBR is a small program that runs every time the computer starts up. It controls the boot sequence and determines which partition the computer boots from. The MBR generally resides on the first sector of the hard disk.
Since the MBR executes every time a computer is started, a boot sector virus is extremely dangerous. Once the boot code on the drive is infected, the virus will be loaded into memory on every startup. From memory, the boot virus can spread to every disk that the system reads. Boot sector viruses are typically very difficult to remove, as most antivirus programs cannot clean the MBR while Windows is running. In most cases, it takes bootable antivirus disks such as a Symantec/Norton AntiVirus (SAV/NAV) rescue set to properly remove a boot sector virus.
Some common boot sector viruses include Monkey, NYB (also known as B1), Stoned, and Form.
Symptoms
A boot sector virus can cause a variety of boot or data retrieval problems. In some cases, data disappear from entire partitions. In other cases, the computer suddenly becomes unstable. Often the infected computer fails to start up or to find the hard drive. Also, error messages such as "Invalid system disk" may become prevalent.
How they spread
Boot sector viruses are usually spread by infected floppy disks. In the past, these were usually bootable disks, but this is no longer the case. A floppy disk does not need to be bootable to transmit a boot sector virus. Any disk can cause infection if it is in the drive when the computer boots up or shuts down. The virus can also be spread across networks from file downloads and from email file attachments. In most cases, all write-enabled floppies used on an infected PC will themselves pick up the boot sector virus.
In the past, setting the computer to boot first from