If you re-install your XP, the Ubuntu Grub Loader in MBR will be overwrite.

You can’t go to Ubuntu anymore.

Boot the Desktop/Live CD. (Use Ubuntu 8.04 or later)

Open a terminal Application > Accessories > Terminal or type Alt-F2 and type

xterm

Start grub as root with the following command :

sudo grub

You will get a grub prompt (see below) which we will use to find the root partition and install grub to the MBR (hd0)
Type the following and press enter:

find /boot/grub/stage1

If you get "Error 15: File not found", try the following:

find /grub/stage1

Using this information, set the root device (fill in X,Y with whatever the find command returned):

(hdX,Y) Harddisk X, partition Y (you may have more than 1 HD)

grub> root (hdX,Y)

Install Grub:

grub> setup (hd0)

Exit Grub:

grub> quit

Reboot (to hard drive).

Grub should be installed and both Ubuntu and Windows should have been automatically detected.