To install OpenCV 2.4.2 or 2.4.3 on the Ubuntu 12.04 operating system, first install a developer environment to build OpenCV.

    sudo apt-get -y install build-essential cmake pkg-config

Install Image I/O libraries

    sudo apt-get -y install libjpeg62-dev 
    sudo apt-get -y install libtiff4-dev libjasper-dev

Install the GTK dev library

    sudo apt-get -y install  libgtk2.0-dev

Install Video I/O libraries

    sudo apt-get -y install libavcodec-dev libavformat-dev libswscale-dev libv4l-dev

Optional - install support for Firewire video cameras

 sudo apt-get -y install libdc1394-22-dev

Optional - install video streaming libraries

 sudo apt-get -y install libxine-dev libgstreamer0.10-dev libgstreamer-plugins-base0.10-dev

Optional - install the Python development environment and the Python Numerical library

    sudo apt-get -y install python-dev python-numpy

Optional - install the parallel code processing library (the Intel tbb library)

    sudo apt-get -y install libtbb-dev

Optional - install the Qt dev library

    sudo apt-get -y install libqt4-dev

Now download OpenCV 2.4 to wherever you want to compile the source.

    mkdir xxx
    cd xxx 
    wget http://sourceforge.net/projects/opencvlibrary/files/opencv-unix/2.4.2/Op...
or 
   wget http://sourceforge.net/projects/opencvlibrary/files/opencv-unix/2.4.3/Op...
    tar -xvf OpenCV-2.4.*.tar.bz2

Create and build directory and onfigure OpenCV with cmake. Don't forget the .. part at the end of cmake cmd !!

    cd OpenCV-2.4.*
    mkdir build
    cd build
    cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local
    -D WITH_TBB=ON -D BUILD_NEW_PYTHON_SUPPORT=ON -D WITH_V4L=ON 
    -D INSTALL_C_EXAMPLES=ON -D INSTALL_PYTHON_EXAMPLES=ON 
    -D BUILD_EXAMPLES=ON -D WITH_QT=ON -D WITH_OPENGL=ON ..

Now compile it

    make

And finally install OpenCV

    sudo make install