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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsA boot sector virus is malware that infects a drive’s master boot record (MBR) or another boot sector—the disk area involved in starting a computer. On traditional BIOS/MBR systems, infected code can run during startup, before the operating system has fully loaded. That location distinguishes it from malware that begins running only after login.
What a boot sector virus infects
A boot sector is at the beginning of a disk or drive. It contains information about the disk and, when the disk is bootable, code used in the startup process. On a traditional BIOS/MBR computer, the MBR can help the system locate and load boot code. A virus that alters this startup-related code may run early in the boot process. NIST describes the category as malware that infects an MBR or the boot sector of a hard drive or removable media.
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The term does not mean any virus that happens to run when a computer starts. It identifies malware by the part of the disk or startup path it infects. The description is most specific to traditional disk boot sectors; modern startup threats can target other components.
How infection and spread worked
Infected removable media
Historically, floppy disks were a common route. If an infected floppy was present when a computer started, its boot code could run and, in some cases, infect the computer. NIST notes that removable media could pose a risk even when the media itself was not bootable, if it was in the drive during startup. From an infected computer, the malware could then copy itself to other floppies accessed by the system.
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Hard-drive boot records
Some examples infected a computer’s hard-drive MBR as well as floppy boot sectors. Microsoft’s historical description of JackTheRipper documents both behaviors, along with stealth and copying to floppies inserted into an infected PC. Microsoft says JackTheRipper is unlikely to run on operating systems later than Windows XP. This is a historical example, not evidence of current prevalence.
Boot sector virus versus bootkit
A bootkit is a broader term for malware that targets early startup or boot components and may start before the operating system. It is related to boot sector viruses, but the terms are not exact synonyms: classic boot sector viruses commonly spread through floppy disks and infected disk boot sectors, while bootkit can describe modern mechanisms that target other parts of the startup chain and conceal malicious activity.
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Because this malware may start before ordinary operating-system protections, it can be harder for software that loads later to inspect. Modern systems use integrity checks at different points in startup to help address that risk.
Possible symptoms—and why they do not prove infection
NIST lists boot-time error messages and inability to boot as possible symptoms of a boot sector infection, and notes that an infection can make a computer inoperable. These signs are not unique to malware. A failed startup by itself does not establish that a boot sector virus is present.
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Form, Michelangelo, and Stoned are examples named by NIST; Microsoft documents JackTheRipper. These are historical illustrations, not a measure of how common such malware is today. The cited sources do not establish a current prevalence figure for classic boot sector viruses.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How modern Windows startup protections help
Microsoft describes several protections that check or record different parts of the startup process. Their availability depends on a computer’s firmware, hardware, Windows version, and configuration; not every device supports or enables every feature.
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- Secure Boot: UEFI firmware checks the bootloader’s digital signature and starts it only if it is trusted or explicitly approved.
- Trusted Boot: Windows checks startup components as the boot process continues.
- Early Launch Anti-Malware (ELAM): An anti-malware driver can load before other non-Microsoft boot drivers and applications.
- Measured Boot: Firmware records information about the boot process so a trusted service can assess the device’s health.
These protections operate at different points and do not amount to a guarantee that every boot-level threat will be prevented or removed. MITRE ATT&CK also identifies boot integrity, TPM technology, and secure or trusted boot as mitigations for threats targeting pre-OS boot components.
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Sources
- NIST SP 800-83 Rev. 1, Guide to Malware Incident Prevention and Handling for Desktops and Laptops (2013)
- Microsoft Learn: Secure the Windows 10 boot process
- Kaspersky: Boot sector virus definition
- Microsoft Security Intelligence: Virus:DOS/JackTheRipper
- MITRE ATT&CK: Bootkit
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