Eh, the search result crapped out for my first link. Just go to the Ubuntu forums and search "dual-boot".
http://ubuntuforums.org/
Here's the deal: Linux has a well-deserved reputation for geekiness, that is, not mainstream. The average user has had difficulty with setup and use. Add to that the fact that there are a great number of distributions (distros) or flavors of Linux, which leads to confusion. Ubuntu is meant to address some of the ease-of-use issues, and make switching over less painful for Windows users. To a great degree, the developers have succeeded.
Download the latest Ubuntu, burn it to disc, set your BIOS to boot from your optical drive, and try it.
I personally would not trust the setup disc to correctly partition my hard drive without backing everything up just in case. I simply put in an extra hard drive into my comp, formatted and made two partitions. I used the free utility provided by the hard drive manufacturer (in this case, Western Digital). Then I cloned my OS drive (XP) to the first partition, making certain I selected the option to make it bootable. Switch the cloned drive to boot drive. Then it's a cinch to install Ubuntu on the second partition. Voila! Dual boot.
The folks at the Ubuntu forums are interested in helping people switch to Ubuntu, there is a weath of information there.
I checked out Vista the other day on a friend's comp. Bleh, it's annoying that it seems so many settings are buried, like you need 5 or 6 clicks to get to a setting, whereas in XP it's 2 or 3. And other things turn me off about it, DRM for instance, but anyway...
Good luck.
[EDIT] BTW, the Beryl desktop beats hell out of Aero. Overall, Ubuntu has a MUCH smaller footprint than Vista. Which means you don't need the latest hardware to run it.