Note that multiple operating systems booting on a single machine is still something of a black art. This document does not even attempt to document the various boot managers, which vary by architecture and even by subarchitecture. You should see your boot manager's documentation for more information.
If you elect to make the hard disk boot directly to GNU/Linux, and you are not installing a diskless workstation, you will be asked to install a master boot record. You will want to do this unless you are using a boot manager or you have another operating system on the same machine. You won't be able to boot into MS-DOS normally on your machine, for instance. Be careful, and see 項8.3. 「DOS および Windows を利用できるようにする」.
If you install a master boot record, the next question will be whether you want to boot GNU/Linux automatically from the hard disk when you turn on your system. This sets GNU/Linux root partition to be the bootable partition — the one that will be loaded from the hard disk.
The standard i386 boot loader is called 「LILO」. It is a complex program which offers lots of functionality, including MS-DOS, NT, and OS/2 boot management. Please carefully read the instructions in the directory /usr/share/doc/lilo/ if you have special needs; also see the LILO mini-HOWTO.
If you can no longer boot into MS-DOS, you'll need to use a MS-DOS boot disk and use the fdisk /mbr command to reinstall the MS-DOS master boot record — however, this means that you'll need to use some other way to get back into Debian! For more information on this please read 項8.3. 「DOS および Windows を利用できるようにする」.