Every system with more than one operating system installed needs a boot manager -- a piece of software that allows users to choose which operating system they wish to use. The boot manager generally resides in the master boot record (MBR) of the hard disk.
The most popular boot managers that come with Linux distributions are LILO (LInux LOader) and GRUB (GRand Unified Bootloader). Lately many distributions, such as Fedora, are encouraging the use of GRUB.
If you install or re-install Windows on a system that already has Linux installed, the Windows bootloader will replace GRUB or LILO in the MBR, so that after you are done installing, you will simply boot into Windows. This doesn't mean your Linux distribution has been erased or uninstalled; it simply means that your current bootloader is unaware of where to find it. It's up to you to tweak the system to get your boot manager back.
If you were using GRUB as your boot manager, you just need to reinstall it. Use the first installation disk of the distribution you had installed. Pop it in your drive and, if necessary, change the boot order in your BIOS to boot off the CD-ROM drive. Reboot, and at the boot prompt, enter:
boot: linux rescue
Let the installer load itself into the rescue mode. When the installer asks you which partition holds the distribution that has to be rescued, select the appropriate partition. The installer will finally mount this partition at /mnt/sysimage and drop you to the shell. To re-install GRUB you need to be 'in' the distribution's environment, so enter the command
chroot /mnt/sysimage, then install GRUB in the MBR with the command
Press Ctrl-D twice to log out and restart. Voila -- there's GRUB. But where's Windows?
Well, now you have to tell GRUB where it can find Windows. Boot into Linux, log in as root, and issue the
fdisk -l command. You should see output similar to the following:
Disk /dev/hda: 40.0 GB, 40020664320 bytes 255 heads, 63 sectors/track, 4865 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/hda1 * 1 255 2048256 6 FAT16 /dev/hda2 256 3545 26426925 f W95 Ext'd (LBA) /dev/hda3 3546 4865 10602900 83 Linux /dev/hda5 256 893 5124703+ b W95 FAT32 /dev/hda6 894 959 530113+ 82 Linux swap /dev/hda7 960 1724 6144831 83 Linux /dev/hda8 1725 2010 2297263+ 83 Linux /dev/hda9 2011 2270 2088418+ b W95 FAT32 /dev/hda10 2271 2780 4096543+ b W95 FAT32 /dev/hda11 2781 3545 6144831 83 Linux
Note the device values corresponding to the partition in which you installed Windows. Edit the file /etc/grub.conf and append the following lines, substituting
hdb and so on). The partition number, specified after the comma below, should be one less than the number you noted. This tells GRUB on which disk and in what partition is Windows installed.
title Windows rootverify (hd0,0) chainloader +1
Save the file and reboot. This time GRUB will show the Windows entry (as specified next to
title in the configuration file).
The process of reclaiming LILO is similar to the one for GRUB. Boot from the first installation disk. Enter the rescue mode as described above. Mount the "to-be-rescued" system under /mnt/sysimage and
chroot to it as shown.
If you lost LILO because you re-installed Windows, just issuing the
/sbin/lilo command should do the trick. If you lost LILO because of a fresh installation of Windows, issue the
fdisk -l command and note the device values next to the partition into which you installed Windows, then edit the file /etc/lilo.conf and append these lines to the end:
Use your device values instead of the
Save the file and issue the /sbin/lilo command. You should see a list of entries that LILO adds to the boot list. Windows (or whatever you specified as the label) should be listed as well.
Now you can reboot, and when the system comes up again, both operating systems should appear on the boot menu.
If you have a choice, which boot manager should you use? Trust me when I say there are as many GRUB vs. LILO battles as there are GNOME vs. KDE, but unlike the latter you can use only one boot manager per machine.
There are various other tricks you can do with boot managers, such as putting up an animated screen instead of simple text. Take some time to go through the configuration files of whichever boot manager you use. Read their documentation to fully understand and appreciate their power and usefulness.