If you’ve installed both Ubuntu and Windows in your computer,your system will be start up through grub and Ubuntu starts as the default OS in grub.you may use Windows more often than Ubuntu and you do not want to select to boot up Windows everytime you start the computer.Here is an easy way to set Windows as default OS.
UPDATE: This tutorial is for Ubuntu 9.04 and earlier, it’s out of date. Read this post for Ubuntu 12.04, 12.10, 10.04, 11.04, and higher.
First,in your Ubuntu open up a terminal window and run following command:
sudo gedit /boot/grub/menu.lst
Find the section in the file like this:
There are four titles match the four grub menu when you start the computer,count from above to below with the numbering starts at 0,in this screen Windows is number 3 and just change the line which says default 0 to default 3.
Another way,in the screen above cut the marked section and paste it to the above of the first title,save and close the file then restart.
Note:For grub2 (ubuntu 9.10 and higher version),see this to set default OS:http://ubuntuguide.net/3-ways-setting-xpvistawindows-7-as-default-bootup-os-in-grub-2