This is my experience in installing dual OS i.e, windows xp and ubuntu linux, at that time my windows broken because many crashes, I tried to reinstall my windows have been broken. after reinstalling, the grub loader was missing because of the PC that has windows installed. for you who have these events, this solution I happen to use ubuntu 9.04, follow the steps ..
1. restart your pc with ubuntu live cd
2. choose "try ubuntu without any change to your computer"

3. type the command:
$ sudo grub

4. after entering the grub prompt, typing:
> find /boot/grub/stage1
(if the command error, then replace it with the command below)
> find /grub/stage1
after the command will appear where you have to install GRUB it were, there would appear (hdx,y)===>(information: where the sign of x, y is a number and y is the partition where ubuntu to install grub and x is your hard disk , is usually worth 0)
note the picture below
5. next, type the command:
> root (hd0,6)
(The location of my partition to install GRUB is (hd0, 6))
6. after that, install the GRUB residing in MBR .. typing:
> setup (hd0)
see picture above if you do it with success...
7.restart your computer:
> quit
$ sudo reboot
1. restart your pc with ubuntu live cd
2. choose "try ubuntu without any change to your computer"
3. type the command:
$ sudo grub

4. after entering the grub prompt, typing:
> find /boot/grub/stage1
(if the command error, then replace it with the command below)
> find /grub/stage1
after the command will appear where you have to install GRUB it were, there would appear (hdx,y)===>(information: where the sign of x, y is a number and y is the partition where ubuntu to install grub and x is your hard disk , is usually worth 0)
note the picture below

> root (hd0,6)
(The location of my partition to install GRUB is (hd0, 6))

> setup (hd0)

7.restart your computer:
> quit
$ sudo reboot
Tags:
Grub loader ubuntu