If you make the mistake of installing Windows after Linux, it will rewrite your MBR, killing GRUB. Some might argue simply installing Windows on your computer is a mistake, but let's fix the MBR and worry about that later. ;-) In my case, my boot partition is on the first partition on the first hard drive. I use Ubuntu in my examples, but any LiveCD should do. First, boot the LiveCD - no need to boot into a GUI. Open a terminal, and become root. In Ubuntu,

sudo -s

works nicely. If you can't remember which partition is your boot partition, try

fdisk -l /dev/sda

OR

fdisk -l /dev/hda

might help jog your memory. Next, we just need to reinstall the bootloader code to the MBR. In my examples, hd0,0 is the first partition on the first disk.

root@ubuntu:~# grub
Probing devices to guess BIOS drives. This may take a long time.
       [ Minimal BASH-like line editing is supported.   For
         the   first   word,  TAB  lists  possible  command
         completions.  Anywhere else TAB lists the possible
         completions of a device/filename. ]

grub> root (hd0,0)

grub> setup (hd0)
 Checking if "/boot/grub/stage1" exists... no
 Checking if "/grub/stage1" exists... yes
 Checking if "/grub/stage2" exists... yes
 Checking if "/grub/e2fs_stage1_5" exists... yes
 Running "embed /grub/e2fs_stage1_5 (hd0)"...  16 sectors are embedded.
succeeded
 Running "install /grub/stage1 (hd0) (hd0)1+16 p (hd0,0)/grub/stage2 /grub/menu
.lst"... succeeded
Done.

grub> quit

That's it - reboot, and you'll be greeted by GRUB again!