12.04 - How to make Grub display after installing Ubuntu on a laptop with Windows OS?

I just installed Ubuntu 12.04.1 on a laptop, but whenever I turn on my computer, it still boots to Windows and GRUB doesn't seem to be working at all.

My partition scheme is as follows:

• /dev/sda1 - Some NTFS partition for Lenovo recovery or something labeled SYSTEM_DRV (this has the Windows boot flag on it)


• /dev/sda2 - My NTFS Windows partition



• /dev/sda3 - My extended partition containing:
  
  
  
  • /dev/sda5 - / partition, ext4, 20 GB
  
  
  • /dev/sda6 - swap partition, linux-swap, 3 GB
  
  
  • /dev/sda7 - /home, ext4, 450 GB
  
  


• /dev/sda4 - Another partition that is useful for Lenovo recovery or something labeled LENOVO_PART.

The point is, I installed Ubuntu 12.04.1 according to this scheme via the LiveUSB environment, but whenever I turn on my computer, the motherboard BIOS screen is shown, and then it jumps right to Windows and GRUB is skipped completely.

What can I do to fix this?