Find a friend who has Windows 98.
Have him make a bootsble floppy disk,
and put FDISK.EXE on it. (usually found
in C:\WINDOWS\COMMAND directory.

Boot from this floppy disk (you may
have to go into setup to set the floppy
as the first boot device).

The FDISK/MBR command should work.
Some times this is the only way to get
it done.

If your BIOS has any virus protection, it must
be disabled, because it will not permit you to write to the
MBR.

If you, by any chance, have a "dynamic disk overlay"
like EZ-BIOS, it is essential that you turn off power, insert
floppy, ignore any instructions from EZ-BIOS, and boot
exclusively from the floppy disk.

EZ-BIOS relocates the MBR, so any time it is loaded, it redirects
reads and writes to the new location, and can confuse you and
your OS, and FDISK.

If you have a Windows install CD, you could boot from it
and it will do the job for you.