You can completely erase your computer's harddrive. That will most certainly get rid of XP. In fact, that is probably the only safe thing to do to completely remove an XP installation.

If you need to save any of the files on the harddrive (before it gets erased) see about plugging the harddrive into another computer and copying any files you want to save to the other computer. Then replace the harddrive in your original computer, and insert any Windows CD. When you restart the computer, it should boot off the Windows CD you inserted and should provide some tools for reformatting the harddrive. If you need more help, post any questions here and someone will gladly provide more assistance.