How can I install Ubuntu if my computer doesn't boot from the CD?

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http://askubuntu.com – I have installed ubuntu alongside windows 7 in a brand new computer and it worked fine for some days. Eventually, Windows showed some problems, but after a safety mode initialization it was working again. Then, Ubuntu cracked and I couldn't use it. After that, I have reinstalled Windows using the recovery CD and now I can't boot from the CD. I put the CD with the Ubuntu image on the drive, turn the computer on, press f12 to choose the drive for the boot, select to boot from the CD and then it automatically starts Windows, ignoring my choice. How can I substitute Windows for Ubuntu? (HowTos)