I saw the edit:
If you want another Windows OS, you have to make a new partition, then install Windows on it.
After that you will have to repair Grub. - https://help.ubuntu.com/community/Boot-Repair - a tutorial on how to do that
Why repair Grub ? - Because Windows boot loader rewrites Grub, and you can no longer access Ubuntu
If you want only Windows OS's on your PC, then you should follow this guide : http://www.howtogeek.com/141818/how-to-uninstall-a-linux-dual-boot-system-from-your-computer/
As far as I understood, you can't access Windows ?
If that's the problem:
Check If Windows XP is in the boot list. If it's not: download Grub Customizer (google it for help with installing it). After you download it you should add your Windows partiton.
(I am dual-booting WinXp and Ubuntu too and I have faced this issue.)
http://www.howtogeek.com/howto/43471/how-to-configure-the-linux-grub2-boot-menu-the-easy-way/ - a link to how to install and work with Grub Customizer)
If your problem is that you see Windows XP in the list, but cannot boot into it, because it gives an error like - Hal.dll not found, the you should do this:
- Boot into Ubuntu
- Open your C: drive
- You should see a file boot.ini and a file boot.ini.bak
- Open both files and copy the content from boot.ini.bak to boot.ini
I will edit my answer as soon as I figure out something that can help you
if things go wrong:
Grub-Customizer made Windows 7 disappear from Gurb2