I'll just say that I don't like having to restore the Vista boot loader after a reinstall or install of XP so if you don't want to reinstall Vista after installing the other 2 then I recommend that you think of something else. A VM emulation would be perfect for experimentation if that is all that you are going to do. You can share files among the host and guest machines and even connect the guest to the internet. When I first had to do this boot loader restore I was still using XP quite a bit (actually more so than Vista) and I had to reinstall XP. I still needed XP on my computer and I didn't want to reinstall Vista. Now I'm running Vista full time but XP is still there. Again it is do-able but it is dangerous and tricky. I don't know how you feel about getting your hands dirty or how comfortable you are with messing around with the OS; that is for you to decide. If you want I can try my best to help you from here and tell you what you need to do but for all practical purposes you are on your own unless you personally know someone who happens to know how to do this kind of thing. The program I use actually runs anther program that is put on every Vista installation to control the boot loader but the problem is that the program is command line only; the program that I found gives you a GUI interface that automates the necessary commands so it isn't as bad otherwise I wouldn't have even tried it. It was tricky enough with the GUI program and is still dangerous.

To answer your file system questions 2k, xp, and vista all use the same version of NTFS as their default file system so their is no problem there and you can set computers to share files over the net.