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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.
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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.
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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.