Set GRUB Default Boot Option

config file = /boot/grub/menu.lst

edit line
## default num
...
default 0

change default number (generally to 4 for std ubuntu/win dual boot)

for karmic koala (9-10), the config file is /etc/default/grub
and change GRUB_DEFAULT=4

in either case, follow up with

update-grub

Source = HowToGeek