A malicious program or malware that modifies a computer’s master boot record. A bootkit is an advanced form of rootkit. When malicious software is attached in this way, it is possible for a malicious program to be run prior to loading the operating system. Because none of the components of a bootkit are stored within the Windows file system, it is impossible for ordinary operating system processes to identify an infection caused by a bootkit. Malicious infections caused by bootkits are becoming less common as more up-to-date computer hardware and operating systems that make use of the UEFI and Secure Boot technologies become more widespread.
Third-party tools may be useless against rootkits that target firmware, boot, or the kernel. To get rid of malware, you may have to delete everything on your computer and start again.