This originally appeared on the Annexe.

From Microsoft’s help files:

If you are running DOS 5.0 or later, Windows 95, Windows 98, or Windows Me, you can use the fdisk utility to try to repair the Master Boot Record. When you use fdisk with the mbr parameter, the Master Boot Record is rewritten to the hard disk without altering the partition table information on the disk. To run this utility, follow these steps:

fdisk /mbr