I installed the ubuntu in this pc in order to do a dual boot with windows.
After installing Ubuntu according to the tutorial it restarts the system, But it doesn't show the GRUB "Boot Selection" menu. My other operating system, Windows 7 starts.
What should I do now?
To address the original question (as InkBlend did), hold the Shift key while booting. That should show the grub menu.
As for GPU lockup, I've seen this with NVIDIA cards on Unity ... hopefully the problem you're seeing is related to what I've seen. I worked around the issue by using GNOME 3. Here's the steps to install GNOME 3 on Ubuntu:
sudo apt-get install ubuntu-gnome-desktop ubuntu-gnome-default-settings
Choose gdm when prompted. Restart. Then next time you're asked to log in, choose GNOME session.
