Dual Boot with Windows

1. Find out the hardware device and partition your windows is installed to:
   

   su -c "/sbin/fdisk -l"

   You will see output like the following, showing windows is installed onto the primary drive:
   

   Disk /dev/sda: 250.0 GB, 250059350016 bytes
   255 heads, 63 sectors/track, 30401 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   Disk identifier: 0x000877eb
   
      Device Boot      Start         End      Blocks   Id  System
   /dev/sda1   *           1        3824    30716248+   7  HPFS/NTFS
   /dev/sda2            3825        3837      104422+  83  Linux
   /dev/sda3            3838       30401   213375330   8e  Linux LVM

   In this case, windows is installed to /dev/sda1 and you can tell this from the "System" format of HPFS/NTFS.
   
2. Find out the device id that grub uses: (make sure you search for the correct device, here we assume sda)
   

   grep sda /boot/grub/device.map

   You will see output like the following:
   

   (hd0)    /dev/sda

3. Configure a grub entry to boot this device. Edit your /etc/grub.conf and add the following:
   

   title Windows
           rootnoverify (hd0,0)
           chainloader +1

   Note how the device to be booted is selected from the information you have gathered in step 1 and 2. We found out that sda is hd0 and that windows is installed to the first partition. The first partition to grub is 0 (yes, it's a zero index) and thus to boot /dev/sda1 the resulting device in grub-land is (hd0,0).
4. Make sure you are aware of what grub entry is set as the default, counting from zero for each "title" entry. The first entry is normally the default (but can be configured with the default=N configuration option) so you will likely want to configure your windows boot option as the last entry.

Windows Installed on a non-Primary Drive:

1. Find out the hardware device and partition your windows is installed to:
   

   su -c "/sbin/fdisk -l"

   You will see output like the following, showing windows is not installed onto the primary drive:
   

   Disk /dev/sdd: 250.0 GB, 250059350016 bytes
   255 heads, 63 sectors/track, 30401 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   Disk identifier: 0x000877eb
   
      Device Boot      Start         End      Blocks   Id  System
   /dev/sdd1   *           1        3824    30716248+   7  HPFS/NTFS
   /dev/sdd2            3825        3837      104422+  83  Linux
   /dev/sdd3            3838       30401   213375330   8e  Linux LVM

   In this case, windows is installed to /dev/sdd1 and you can tell this from the "System" format of HPFS/NTFS.
   
2. Find out the device id that grub uses: (make sure you search for the correct device, here we assume sdd)
   

   grep sdd /boot/grub/device.map

   You will see output like the following:
   

   (hd2)    /dev/sdd

3. Configure a grub entry to boot this device. Edit your /etc/grub.conf and add the following:
   

   title Windows
           map (hd2) (hd0)
           map (hd0) (hd2)
           rootnoverify (hd2,0)
           chainloader +1

   Note how the device to be booted is selected from the information you have gathered in step 1 and 2. We found out that sdd is hd2 and that windows is installed to the first partition. The first partition to grub is 0 (yes, it's a zero index) and thus to boot /dev/sdd1 the resulting device in grub-land is (hd2,0). Make sure to note, you must re-map your drives or NTLDR will not correctly boot. More information on this: here.
   
4. Make sure you are aware of what grub entry is set as the default, counting from zero for each "title" entry. The first entry is normally the default (but can be configured with the default=N configuration option) so you will likely want to configure your windows boot option as the last entry.