GRUB_DEFAULT=0

GRUB_DEFAULT="Windows Vista (loader) (on /dev/sdc1)"

sudo update-grub

user@YourComputer:~$ grep menuentry /boot/grub/grub.cfg
menuentry 'Ubuntu, with Linux 2.6.35-31-generic' --class ubuntu --class gnu-linux --class gnu --class os {
menuentry 'Ubuntu, with Linux 2.6.35-31-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os {
menuentry 'Ubuntu, with Linux 2.6.35-30-generic' --class ubuntu --class gnu-linux --class gnu --class os {
menuentry 'Ubuntu, with Linux 2.6.35-30-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os {
menuentry "Memory test (memtest86+)" {
menuentry "Memory test (memtest86+, serial console 115200)" {
menuentry "Windows NT/2000/XP (loader) (on /dev/sda1)" {
menuentry "Ubuntu, with Linux 2.6.32-34-generic (on /dev/sdb1)" {
menuentry "Ubuntu, with Linux 2.6.32-34-generic (recovery mode) (on /dev/sdb1)" {
menuentry "Ubuntu, with Linux 2.6.32-33-generic (on /dev/sdb1)" {
menuentry "Ubuntu, with Linux 2.6.32-33-generic (recovery mode) (on /dev/sdb1)" {
menuentry "Windows Vista (loader) (on /dev/sdc1)" {