how can I install Windows xp over ubuntu

¿? If you wish to remove ubuntu and install XP just boot with your original windows cd, and install it. It's possible that windows won't start after the first reboot, there are several ways to repair this, it depend on your computer skills, if you are not familiar with partiton and boot managers the "easy" way whould be to start your pc with the windows cd, open the restore console, and type fixboot or from a "windows start-up disk" type fdisk -fixmbr (be aware that this commands may very well delete all your hard drive, but you'll have to do it anyway in order to remove ubuntu completely).
I don't know why you wish to install xp (and I know is none of my business) but please note that ubuntu (linux in general) is far more stable, reliable and secure, so don't let a bad experience ruin your views of open source.
Have a nice weekend!