Hi jawad. Welcome to NST.
If you're talking about using Wubi to install Ubuntu on the same partition as XP, then the Wubi installer should automatically add Ubuntu to the boot menu. EasybCD is a tool for editing the Vista/7 BCD, which OSes prior to Vista/7 (such as XP) do not use. XP uses the NTLDR bootloader, which reads from a boot.ini file. Wubi will automatically edit this boot.ini file to add an entry to load the file Ubuntu needs in order to boot properly from the root.disk.
