William Wilson...will you help me?

I have been studying your posts most of the late afternoon and into the evening today. I am only leary of one step in the process of downgrading my pc......Gateway 6847 with pre-installed Vista......to Windows XP SP2.
The step on the dual boot.....where I need to put in the Vista DVD----ugh! No Vista DVD.....I was considering all sorts of options.....some you would downright probably laugh at....but I am a thinker.....anyway.........can I find those Vista GUI drives 1) on the Gateway that is up an running with Vista and copy to a Thumb Drive....then go into the BIOS and change the Boot Order to read that thumb drive with the GUI files on it..........or......2)download the Vista GUI files from the Internet.....I do not have a floppy drive on the computer. I am just getting brain dead with the information overload today............and want to bravely attempt this. You have given such thorough information............I was hopeful to get a solution from you directly. But anyone who is willing to offer a little support....I thank you in advance! I love this nerdy stuff!