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There are three approaches to doing this, and which one is best for you depends on your goals in installing and using Ubuntu:
2 and 3 will both create a dual-boot Windows/Ubuntu environment; however, 2 will require you to resize your Windows partition (which I think the Ubuntu installer will guide you through nowadays), while 3 will be slower than a normal Ubuntu installation (Wubi installs Ubuntu to a giant file on your hard drive), though it's much easier.
1 is certainly the easiest, but you'll need to be willing to part ways with your Windows installation; I'd back up the Windows product key and any files you have on there before installing.
I should clarify that the Ubuntu website provides full instructions on how to create the install disk, boot it, and go forward with the installation:
That page not only provides the link to download the installation image, but also provides links that detail how to burn it to a DVD, flash it onto a thumb drive, grab the Windows Installer and install via Wubi instead, etc.