Disk Concepts and Troubleshooting

It is always important to take precautions to protect your computer and the data on it from viruses. Many computer viruses exploit the disk structures that your computer uses to start up by replacing, redirecting, or corrupting the code and data that start the operating system.

MBR Viruses

MBR viruses exploit the master boot code that runs automatically when the computer starts up. MBR viruses are activated when the BIOS activates the master boot code, before the operating system is loaded.

Many viruses replace the MBR sector with their own code and move the original MBR to another location on disk. Once the virus is activated, it stays in memory and passes the execution to the original MBR so that startup appears to function normally. Some viruses do not relocate the original MBR, causing all volumes on the disk to become inaccessible. If the active, primary partitions listing in the partition table is destroyed, the computer cannot start. Other viruses relocate the MBR to the last sector of the disk; if that sector is not protected by the virus, it might be overwritten during normal use of the computer, preventing the system from being restarted.

Boot Sector Viruses

As with the master boot code, the boot sectors executable code also runs automatically at startup, creating another vulnerable spot exploited by viruses. Boot sector viruses are activated before the operating system is loaded and run when the master boot code in the MBR identifies the active, primary partition and activates the executable boot code for that volume.

Many viruses update the boot sector with their own code and move the original boot sector to another location on disk. Once the virus is activated, it stays in memory and passes the execution to the original boot sector so that startup appears normal. Some viruses do not relocate the original boot sector, making the volume inaccessible. If the affected volume is the active, primary partition, the system cannot start. Other viruses relocate the boot sector to the last sector of the disk. If that sector is not protected by the virus, it might be overwritten by normal use of the computer, rendering the volume inaccessible or preventing the system from restarting, depending upon which volume was affected.

How MBR and Boot Sector Viruses Affect Windows 2000

A computer can contract an MBR or boot sector virus by one of two common methods: by starting up from an infected floppy disk; or by running an infected program, causing the virus to drop an altered MBR or boot sector onto the hard disk.

The function of an MBR or boot sector virus is typically contained once Windows 2000 has started. If a payload is not run during system startup and the virus preserved the original MBR or boot sector, Windows 2000 prevents the virus from self-replicating to other disks.

Windows 2000 is immune to viruses infecting these disk structures during normal operation, because it only accesses physical disks through protected mode disk drivers. Viruses typically subvert the BIOS INT 13h disk access routines, which are ignored once Windows 2000 has started. However, Windows 2000 computers that are multiple-booted with MS-DOS, Windows 95, or Windows 98 can become infected when Windows 2000 is not running the computer.

If a multiple-boot computer on which Windows 2000 has been installed becomes infected by an MBR or boot sector virus while running another operating system, Windows 2000 is vulnerable to damage.

Once the protected mode disk drivers have been activated, the virus cannot copy itself to other hard disks or floppy disks because the BIOS mechanism on which the virus depends is not used for disk access. However, viruses that have a payload trigger that executes during startup are a threat to computers that are running Windows 2000 because the trigger process is initiated before the control during the computer startup process passes to Windows 2000.

Treating an MBR or Boot Sector Virus Infection

To remove a virus from your computer, use a current, well-known, commercial antivirus program designed for Windows 2000, and update it regularly. In addition to scanning the hard disks in your computer, be sure to scan all floppy disks that have been used in the infected computer, in any other computers, or with other operating systems in an infected multiple-boot computer. Scan them even if you believe they are not infected. Many infections recur because one or more copies of the virus were not detected.

If the computer is already infected with a boot sector virus when Windows 2000 is installed, standard antivirus programs might not completely eliminate the infection because Windows 2000 copies the original MS-DOS boot sector to a file called Bootsect.dos and replaces it with its own boot sector. The Windows 2000 installation is not infected, but if the user chooses to start MS-DOS, Windows 95, or Windows 98, the infected boot sector is reapplied to the system, reinfecting the computer. Antivirus tools that are not specifically designed for Windows 2000 do not know to check Bootsect.dos for viruses.


Microsoft provides a customized antivirus tool that can be used for these types of viruses. AVBoot is located in the \Valueadd\3rdparty\Ca_antiv folder of the Windows 2000 Setup CD. Insert an empty, high-density, 3.5-inch floppy disk, and use Windows 2000 Explorer to locate and double-click Makedisk.bat to create a startup floppy disk that automatically runs AVBoot.

AVBoot scans the memory as well as the MBR and all boot sectors of every locally installed disk. If a virus is found, it offers to remove the virus.



Whether you use a third-party antivirus program or AVBoot, be sure to regularly update the virus signature files. Once you install an antivirus program, immediately update the signature files, usually through an Internet connection. Check with the software manufacturers documentation for specific instructions. AVBoot includes update instructions in the installation folder and on the AVBoot floppy disk.

It is extremely important that you regularly update your antivirus program. In most cases, antivirus programs are unable to reliably detect and clean viruses of which they are unaware. False negative reports can result when using an out-of-date virus scanner. Most commercial antivirus software manufacturers offer monthly updates. Take advantage of the latest download to ensure that your system is protected with the latest virus defenses.

Fdisk /mbr command

Do not depend on the MS-DOS command Fdisk /mbr , which rewrites the MBR on the hard disk, to resolve MBR infections. Many newer viruses have the properties of both file infector and MBR viruses, and restoring the MBR does not solve the problem if the virus immediately reinfects the system. In addition, running Fdisk /mbr in MS-DOS on a system infected by an MBR virus that does not preserve or encrypt the original MBR partition table permanently prevents access to the lost partitions. If the disk was configured with a third-party disk management program, running this command eliminates the program overlay control and you cannot start up from the disk.



Running Fdisk /mbr in MS-DOS overwrites only the first 446 bytes of the MBR, the portion known as the master boot code, leaving the existing partition table intact. However, if the signature word, the last two bytes of the MBR, has been deleted, the partition table entries are overwritten with zeroes. If an MBR virus overwrites the signature word, access to all partitions and logical volumes is lost.

Fixmbr command

The Recovery Console, a new troubleshooting tool in Windows 2000, offers a feature called Fixmbr . However, it functions identically to the Fdisk /mbr command, replacing only the master boot code and not affecting the partition table. For this reason, it is also unlikely to help resolve an infected MBR.

For more information about the Recovery Console, see Troubleshooting Tools and Strategies in this book.