Can't install Ubuntu 12.04 64 bits

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http://askubuntu.com – I have just bought an hp pavilion DV6 ( intel i7 processor + 6G of RAM) running windows 7( 64 bits). I downloaded Ubuntu 12.04 64 bits and correctly burned it to a DVD. My problem starts when I try to boot the computer from DVD as a first step to install Ubuntu. In fact, the home page of Ubuntu is shown without further prompts ( no button of install of try it before install). I tried the process many times without results. Why couldn't I install the 64 bits version of Ubuntu 12.04 ( knowing that my laptop has a 64 bits architecture)?? I would really like to install the 64 bits version to be ab (HowTos)