FIXMBR

The FIXMBR is one of several Windows Recovery Console commands.  After you select the operating system and logon 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.

FIXMBR

The FIXMBR website has some additional articles you might find useful.