By installing Ubuntu with the option "Install side by side with Windows 7" for a dual-boot installation you will replace the MBR and by this also your Easy BCD bootloader by Grub2.
Before you install Ubuntu you will have to delete the partition that holds Windows XP to make space for the Ubuntu OS. No need to format or partition at this point, the installer will do that for you later.
When your disk is fairly standard partitioned then Grub2 will detect any exisiting Windows 7 installation, and will offer a menu choice at boot to choose the OS you want to run. Thus, if you have a spare partition you may also install Ubuntu as triple-boot with Windows 7 and Windows XP.
In theory it is also possible to chainload Grub from Easy BCD but this is a rare installation type, and thus not really recommended.
In case your Windows Boot Record fails to start Windows 7 you may have to rebuild it with the help of a Windows repair or installation CD. Thenafter a reinstallation of Grub2 is needed for booting Ubuntu.