Questions Regarding Windows->Ubuntu Switch

First of all, I'm sorry if similar questions have been answered in the past, I read all of the ones that popped up as similar to my inquiry, but none of them answered what I was looking for.

First - If I run a dual boot of Ubuntu and Windows, will they both be able to access all the files on my hard drive? Specifically, will Ubuntu be able to access files I have created/downloaded while using windows (video, audio, etc)?

Second - In looking at the way I use my computer, I think that don't really have a need for windows, considering the programs I use most. However, there are one or two applications which I use that are Windows only. I know there are emulators that exist for running these types of applications on a Linux system, but if my application, uses a high amount of resources, and uses lots of high resolution graphics and audio files, should I expect a noticeable decrease in performance and/or quality?

Third - If I got through everything and decided I had no need to rely on windows for anything, thus deciding to strictly use Ubuntu, can you tell me how I would do the following =

I would want to get rid of Windows and all related applications, however, there are some files I would not want to remove from my hard drive including audio, video, documents, and Thunderbird files. How do I keep all of these files and get rid of everything else? Should I make a partition, then put all the files I want to keep in it?

Fourth - Japanese support is a necessity for me, as I do work in Japanese on the computer. I have read how to do this and configured it on the Ubuntu livecd, and it works fine, but are there any possibilities that the use of Ubuntu could inhibit my ability to interact with users of other OSs through email, instant messenging, or otherwise?

Thanks in advance for your time and help. It is greatly appreciated.