I had dual operating systems but when i re-install windows after that my ubuntu has been disappeared

At pc startup after BIOS screen you should view the "GRUB" row...

If you view it press ESC key... and select from item...

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Thank you

I too have the same the problem...i have installed Ubuntu with XP...but the boot loader is not showing up...only the XP is booting...the line is even available in boot.ini of XP to..Please me out with this..

Please tell if you can see the GRUB single line...
If you can't view GRUB row i think you must consider to reinstall GRUB using the live Ubuntu install CD

Here detailed howto https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows

Be sure to have read the previous howto.

Here a short howto
Boot from live cd and open a Terminal from the menu Applications->Accessories->Terminal and type:

sudo grub

give your user password when requested, you don't see nothing when you type it, then press enter.

Then type, (don't type the "grub>" part it is returned by GRUB shell") :

grub> find /boot/grub/stage1
(hdx,y)
grub> root (hdx,y)
grub> setup (hdx)

Substitute (hdx,y) with your system founded values, returned by the find /boot/grub/stage1 command

then restart system

If this works then you must add the Window section by edit the /boot/grub/menu.lst GRUB file
Boot from installed Ubuntu, open a terminal from the menu Applications->Accessories->Terminal and type:

sudo gedit /boot/grub/menu.lst

and append a Windows section rows like this:

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

Substitute the hd0,0 with your Windows partitition value (where Windows is installed):
hd0,0 where hd0 is the first disk, 0 is the first partition on the first disk
hd0,1 where hd0 is the first disk, 1 is the second partition on the first disk
hd1,0 where hd1 is the second disk, 0 is the first partition on second disk
hd1,1 where hd1 is the second disk, 1 is the second partition on second disk

Save and exit and type:

sudo update-grub

Then restart the system
Thank you

While installing Ubuntu through dual booting, using WindowsXP, I made four partitions as under:
Device Boot System
/dev/sda1 FAT32
/dev/sda2 FAT32
/dev/sda3 * Linux
/dev/sda4 swap
Hoping that sda1 and sda2 will be Windows XP with sda1 as recovery disk in lieu of original recoivery disk H and sda2 for other disk C. At the end on rebooting I now find only ubuntu installed and no XP. On ubuntu desktop I have got sda1 and sda2 with ubuntu documentation, there is no sign of sda3 or sda4 and I do not get an alternative for Windows XP on booting.
I tried the above command and except for knowing stage1 is hd0,2, which I have setup as instructed; I have not been successful in getting windows back. On booting, I only get Ubuntu options.

I cannot see any thing related to UBUNTU. Straight away my PC goes to Windows xp

Still I have not resolved my problem any can tell me please