Help with Dual Boot

Hey all,
Before, when i started my pc, i could choose between:
Older version of windows
Ubuntu
Windows 7

Later i got some problems with Windows XP, so i reinstalled XP.
Now when i start up my pc, it automatically starts XP, without the Dual boot screen.

How can i bring it back?

Thanks in advance.

I think you need to re-install grub.