I was wondering if I will be able to download Ubuntu onto a DVD. If so, will doing so automatically replace Windows as my OS? I'm downloading it using someone else's computer and I don't want to accidentally download Ubuntu onto their computer. Basically what I'm asking is will downloading Ubuntu on a DVD only burn it onto the disc or will it replace Windows as the computer's operating system? Another question; is a 4.7gb disc going to be enough space for the operating system?
Sounds like we need to clarify some concepts for you.
By default, "downloading Ubuntu" using a Windows computer will not automatically replace Windows OS. What you will be doing is downloading a file (a large one at about 4GB) that can be burned onto a blank DVD disc. This DVD can then be used to install Ubuntu onto any computer that 1) supports booting from a DVD disc, and 2) supports the version of Ubuntu you download. BUT, you have to put the DVD in that computer AND boot from it AND tell Ubuntu to install AND tell it to replace Windows.
So in short: No, if you just download the latest Ubuntu Desktop file, it will not replace Windows on your friends computer.
Good luck, and be sure to check out the Ubuntu community documentation. We are a pretty decent group with lots of help for newcomers. This site and UbuntuForums.org are a good place to start, as well as Ubuntu.com of course.