The Fixboot is a Recovery Console command that writes a partition boot sector to a system partition that you can specify.
Boot Sectors
Boot sector is a sector on a hard drive or data storage device that contains code for booting a program stored on some other part of the storage media. The computer BIOS can be set to select any of several boot devices and then copies the first sector from the boot device to address location 0×7C00.
Different kinds of boot sectors can be encountered on computer hard disks and other storage devices. A master boot record (MBR) is the first sector of a data storage device that has been partitioned. The MBR sector contains code to locate the active partition and start its Volume Boot Record.
Volume Boot Record (VBR)
A volume boot record is the first sector of a data storage device that has not been partitioned. It contains code to load and start an operating system installed on that device or within that partition.
Valid boot sectors have a two-byte hex word 0xAA55, called the boot sector signature and must exist at the end of the sector. If the signature is not present either the BIOS or MBR code will report an error message and halt any OS startup process.
The computer BIOS does not distinguish between VBR and MBR. The firmware will only load and runs the first sector of the storage device. floppy devices or USB flash drives will use a VBR to boot. Hard drives use an MBR. The MBR has the partitioning information and will start the VBR which will load the operating system.
Boot Sector Viruses
Since the code in the boot sector is first executed automatically the boot sectors have been targets for computer viruses. As a warning computer BIOS now have included a feature to prevent writing over the boot sectors without user permission.
FIXMBR
FIXMBR is a Recovery Console command that writes a new MBR to the first sector of a hard drive. This command can be used to correct a corrupted or infected MBR. The fixboot command write the partition boot sector.
The FIXMBR website has some additional articles you might find useful.
you can type FIXMBR at the prompt and the MBR (master boot record) is rebuilt and replaced with a new MBR file. This can usually correct some of the startup problems with a corrupted or infected MBR record. Since the MBR record is the first 512 bytes read on the boot hard drive it is also a target for many viruses. If the MBR gets infected the virus can move into memory and control all other boot processes and disable any anti-virus programs. Most anti-virus programs like AVG will detect and stop viruses, but if you boot the computer to an infect device, such as, a floppy or CD, it can infect the MBR. Just rebooting the computer to Recovery Console and using FIXMBR will remove the virus and correct the problem. One WARNING that should be mentioned is that dual boot systems where you have more than one bootable operating system on a hard drive can be affected so that you lose the ability to boot the alternate operating system. You might want to backup the MBR record first just so you can restore it if needed. I also have a FIXMBR YouTube video you might find helpful.