mickeddie: the version you want will depend on what your cpu is. If you have a 64 bit cpu, you can use the 64bit version but there is also a lot of compatibility issues using older programs under a 64bit OS especially with games. You may just want to try the 32bit version first.
ISO files are disk image files meaning when you burn them to a DVD they will be a bootable DVD that you put into your drive and autorun or doubleclick to execute the install.
You can install Vista without the key; just make sure you get one before the trial tie (14 days) is up. I suggest you go to the Vista site and get it because their program is now open and you can get a legal Vista key.
I installed the 64bit version. Most of my old 32bit programs work pretty fine... although I have to disable Aero for some because they are not compatible with the new GUI.