On a PC which has Ubuntu installed you initially see the GRUB boot manager. To edit the default behaviour (i.e. which OS is booted without interaction) you must edit the menu.lst file
sudo gedit /boot/grub/menu.lst
Find this section of the file:
## default num
The value “0” boots the OS which is at the top of the list.
If you already had Windows on the system before installing Ubuntu and you want to boot Windows by default you will need to change that number to “4” then save and reboot your machine. You should go into Windows instead of Ubuntu automatically.