This question already has an answer here:
How do you install ubuntu on a pre-installed windows7 system which usually comes with the max of 4 primary partitions already used up by the windows 7 installation. As these systems do not come with a windows system disk you cannot simply re-install windows if you mess up. So getting it right first time is critically important. I have seen some great instructions on www.avoiderrors.net but none deal with this specific but very common situation encountered by new users.