Already Window XP has been installed and then Linux has been installed .But now I uninstall the Window XP but I can't . How is it possible??

You can't "uninstall" an operating system.

I went through that. If you format the drive that XP is on you may kill the boot manager (usually GRUB) as well.

Check here: removing xp after installing ubuntu - Google Search

Removing XP is as simple as formatting the partition it occupies. You will not mess up your bootloader because formatting the XP partition doesn't touch the MBR. After removal you may have to update GRUB in order to access the partition you just cleared.

For the most part it is easier to setup a dual boot system by installing the Linux OS after the windows OS is installed and running. You can use a drive cloning utility to clone the drive as is and then format and install Windows back from the cloned archive before setting up the second Operating system. Also running Linux as a virtual machine with Virtual Box or VM ware works well

There is another way to do it -- wipe the drive -- Do a clean install of Linux