Howto Set Windows as Default OS when Dual Booting Ubuntu
You need to edit /boot/grub/menu.lst file
sudo vi /boot/grub/menu.lst
Look for the following option
Save and exit the file and reboot your system.
You will need to change that number 0 to match the Windows boot section.In most cases 4 is a default dual-boot configuration and in one my friend's laptop this value is 5 so you need to check this and enter the value.The blocks at the bottom of the file match the items in the menu and the numbering starts at 0