If your Linux OS was installed before you Windows OS, Linux Grub boot loader will not show Windows OS in his menu. To back or to show your missing Windows OS option in Grub boot menu you need manual editing of the Grub boot menu file. Next simple steps will show you how to do that:

1. Boot into linux.
2. Open terminal.

3. We must make backup file in case if something go wrong, so we can make a recovery from backup file. Make backup of menu.lst with this command:
sudo cp /boot/grub/menu.lst /boot/grub/menu.lst.backup

4. Now, open menu.lst with:
sudo gedit /boot/grub/menu.lst

You will see all active boot OS options: (I use Ubuntu linux, but this list is the same in all linux distributions):

title Ubuntu 7.10, kernel 2.6.22-14-generic
root (hd1,0)
kernel /boot/vmlinuz-2.6.22-14-generic root=UUI...
initrd /boot/initrd.img-2.6.22-14-generic
quiet

title  Ubuntu 7.10, kernel 2.6.22-14-generic (recovery mode)
root (hd1,0)
kernel /boot/vmlinuz-2.6.22-14-generic root=UU....
initrd /boot/initrd.img-2.6.22-14-generic

title Ubuntu 7.10, memtest86+
root (hd1,0)
kernel /boot/memtest86+.bin
quiet

5. Enter the folowing code on the top of this list. This will make Windows primary boot OS. If you want Windows OS as a second option write folowing lines after first ubuntu option:

title  Windows XP or Vista
root  (hd0,0)
makeactive
chainloader +1

6. Save menu.lst file.
7. Restart Computer!

Tags | Howto, Linux, Tips and Tricks, Windows