All you have to do is go to http://www.ubuntu.com/getubuntu/download and follow the directions. I downloaded Ubuntu, burned it to CD, and rebooted with the CD in the drive. I did not have to get a second hard drive or worry about it messing with my Vista Home Premium installation.
With Ubuntu (as with most distros with a Live CD install) all you need to do is pop in a disc, boot, and click through the menus. The dual boot is set up perfectly by default, you don't even have to think about it. I've done this with Ubuntu, Debian, PC Linux OS, Freespire, and Xandros on my Vista Home Premium machine and they all worked that way.
If you are paranoid, then you should back up your PC. As cheap as hard drives (USB or internal) are these days, there really is no excuse to not have a full back up of your system. It's too easy. I use Acronis True Image, but I've heard good things about Norton Ghost as well.
Regadless, you don't need Wubi or VMWare, or any virtual anything, a straight install with a dual boot set up is the default on a typical Live CD Linux install and it works even with Vista.