What command do you use to fix problems with the mbr?

To fix problems with the Master Boot Record (MBR) on a Windows system, you can use the command bootrec /fixmbr in the Command Prompt. This command can be executed from a Windows installation or recovery media. Additionally, you may also use bootrec /fixboot and bootrec /rebuildbcd to address more comprehensive boot issues. Always ensure to back up important data before making changes to the MBR.