The FAQ mentions "fdisk /mbr" and that "It will not write the MBR to another drive NO MATTER WHAT OTHER PARAMETERS YOU SPECIFY."
This is true. However, "fdisk /cmbr <drive>" will rewrite the Master Boot Record of any drive. If memory serves <drive> is a number, and starts at 1 (consistent with the menus) so 1 = drive 0 = "C:". But don't trust me--make sure the other possibility is safe territory before you do it.
(Why they didn't just add this functionality to the /mbr switch is one of those stupid mysteries of the micro,soft mentality (right up there with all their extended-partition-with-sub-partitions and nut-case-mounting-order nonsense--which has surely caused many to suffer a great deal of mechanical hair-loss).) (But i digress.)