Boot Camp Assistant says "The disk cannot be partitioned because some files cannot be moved." What can I do?

I am trying to install Windows 7 Ultimate on my MacBook Pro that runs OS X Lion. I have read through a lot of the other threads and I think I understand that the I need a clean, defragged part of the hard drive to do this on. Boot Camp Assistant tells me to "Back up the disk and use Disk Utility to format it as a single Mac OS Extended (Journaled) volume. Restore your information to the disk and try using Boot Camp Assistant again." That's all good and everything, but I don't have any idea how to do that. I don't want to risk losing any of my data and I don't want to download any "freeware" to try and help me accomplish this task. I would like a step by step guide on how to make this work. Also, is there anyway that I could have prevented this error? I don't understand why Boot Camp Assistant is even on my MacBook Pro if everyone that uses it will get the same error. I have done nothing but taken it out of the box and started using the Apple software that came installed on it. If there is not a simple way to do this in the Disc Utility, then I don't want to do it at all. I really don't like the idea of putting all my data on an external hard drive just to have this done.