If you install Ubuntu over XP and you set a dual-boot, Ubuntu is the default operating system in the Grub loader.
To set Windows as the default OS again, you have to boot into Ubuntu and open the file menu.lst for edit with this command:
sudo gedit /boot/grub/menu.lst
Find this section of the file and edit the "default 0" line:
Change this value to 4, save the file and reboot your computer.
When booting, if you don't choose any operating system, you should go into Windows automatically.
