A very good article on how the MBR works can be found here.

Historically, in Microsoft-land, the MBR has been repaired using a DOS boot floppy and the command fdisk. However, the command fdisk is incompatible-with-Windows NT/2000/XP.

In walks "MBR Fix". It's free and it works from Windows NT/2000/XP/2003 command prompt, and so is perfectly suited for use in removable operating systems (ERD Commander, BartPE, WinPE, etc.) You can download it here.