ohh god,, really?now shall i format da xp partition whn im in a boot menu?
Operating systems use what is called a "boot loader" to determine what OS to boot. This boot loader exists at the very beginning of the hard drive and only one boot loader can be installed at a time. This is where you select which operating system to boot.
Since Windows XP is so old, it most likely does not understand the Windows 7 boot loader, so it overwrote it with its own without adding Windows 7 as a boot option. If your computer is new and you installed a 64 bit version of Windows 7, Windows 7 may have formatted the hard drive with a GPT partitioning scheme which Windows has no idea how to read. If you were careless in installing Windows XP then you may have completely destroyed your Windows 7 installation.
There is no reason to install Windows XP as Windows 7 has almost flawless compatibility with Windows XP applications.