I installed ubuntu 12.04 with using DVD beside my Windows 7. Installation was completed. Then it asked for restarting the system. After that system goes to Windows 7 without asking me to choose between Windows 7 and ubuntu..
You should use your startup cd/dvd to start an ubuntu live session, and then either install boot-repair and run it, or follow the detailed steps from here: Recovering GRUB after installing Windows 7?