Ubuntu (12.04 and other versions seem to have this issue) installed for dual boot but immediately boots into Windows without displaying the menu options to boot into eg. Ubuntu in the first place? The below was tested during a support call from South Africa to Namibia, hopefully it works for you too:
1. Boot from a live cd, open terminal and enter "sudo su -" to become root user, to check if your root use "whoami"
2. Use fdisk -l to locate your Linux partition, eg. /dev/sda6 (could be /dev/sdb/c etc)
3. Make a directory: mkdir /linfix
4. Mount the file system: mount /dev/sda6 /linfix
5. Use nano to edit: nano /etc/default/grub (change GRUB_TIMEOUT=0 to 10 (second) or -1 (indefinite))
6. Save the file in nano using: ctrl+o <enter> ctrl+x
7. Reinstall grub: grub-install --root-directory=/linfix/ /dev/sda
8. Reboot if no errors encountered in 7, if encountered Google em!