FIXMBR is a Windows XP Recovery Console command. This command is used to repair or correct a corrupted or infected MBR. The MBR is the master boot record which is the first sector on a hard drive. Since it is the first sector read it is prone to virus infections. Also if it becomes corrupted it prevents access to your hard drive and your operating system. Therefore a tool in the Windows XP Recovery Console has been provided to quickly fix the MBR. You can also use FDisk MBR which accomplishes the same thing in DOS. There is an article explaining Fdisk MBR in more detail at Fdsik MBR. Also another article that takes about FIXMBR in more detail at FIXMBR. The Recovery Console also has many other DOS like commands that can be used to help maintain and repair other files and folders on your hard drive. Many of the commands that can be accessed, such as, copy, del, rd, md, chkdsk, fixmbr, and many others, can be very helpful to keep Windows running efficiently.
The Computer Repair Training website has some additional information you might find helpful.