If you are installig Ubuntu inside windows, you may see that Ubuntu is set as default OS in boot menu. If you need to change the default OS to windows, you need to edit GRUB. In Ubuntu Lucid, you can do this as follows.
First open Terminal and type sudo gedit /etc/default/grub. This will open the Grub.cfg file. It will look like below.
# If you change this file, run ‘update-grub’ afterwards to update
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
# Uncomment to disable graphical terminal (grub-pc only)
# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo’
# Uncomment if you don’t want GRUB to pass “root=UUID=xxx” parameter to Linux
# Uncomment to disable generation of recovery mode menu entries
# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE=”480 440 1″
Here you need to change the line GRUB_DEFAULT=0. Change the value of 0 to 4 or 5. This depends on the position of your desired OS in boot menu. Suppose you need Windows XP as default boot and it is in 5th position in boot menu, you need to give the value as 4. That is number of position-1. After doing this, save and close the page. Next run the command sudo update-grub to update the Grub. Close terminal and on next restart you can see your default operating system selection is changed. Though we change the value, only default cursor selection is changing. Default boot selection time is 10 seconds. If you need to change it edit the line GRUB_TIMEOUT=10. Remember, whenever you change the Grub, always run update-grub command.