After installing Ubuntu (dual booting with Windows 7), the Grub boot loader sets Ubuntu as the default OS.  To set a new default value, follow the steps below:

1. As root, edit the grub configuration file:
    
   sudo gedit /etc/default/grub
    
   
    
2. Set the GRUB_DEFAULT value.  The list of items you see at the grub menu begins at 0, so a new install is usually 4 for Other/Windows, or 6 if Ubuntu has been updated.  I also like to bring the GRUB_TIMEOUT value down to 5 seconds:
    
   GRUB_DEFAULT=6
   #GRUB_HIDDEN_TIMEOUT=0
   GRUB_HIDDEN_TIMEOUT_QUIET=true
   GRUB_TIMEOUT="5"
   GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
   GRUB_CMDLINE_LINUX=""
    
3. Save changes and then run update-grub to finalize the changes: