If you have a copy it is possible to install it without activation for 30 days, and then 'renew' the trial up to 3 times for a total of 120 days. However, Microsoft does not offer Vista for download unless you are purchasing it. You can, however, go to Best Buy, Circuit City, etc. and try it out, or use a friend's computer that has it installed.
To answer your other questions, yes, it is possible to install Vista while leaving XP installed, and yes, you can alternate between two or more operating systems on the same computer.
I have read that there is a download that will let you try vista free for 30 days. Where do I get it? Can you run vista and still keep xp installed? Is it possible to run two operating systems on your computer?