There are many, many tutorials out there. Pick the distribution you want and search for installation guides for it. (K)Ubuntu is the most popular and has the largest community, but the others are good choices, too.
You'll find there are several ways to install Ubuntu. The installation guide offers a decent overview, but you could also try Wubi (the Windows Ubuntu installer).
You might find some kinks while trying to install one of the distros, so pick a few to try. I'd recommend Ubuntu/Kubuntu, Fedora Core, or OpenSuSE, in that order. Ubuntu and Fedora work best with Gnome, a fairly simple desktop environment. Kubuntu and OpenSuSE work best with KDE, which is a bit flashier. They also have different selections of applications that come bundled, although you can install any of them.
Dual-booting is extremely common, so it's addressed by nearly every tutorial you'll find. However, if you can afford it, consider buying another hard drive instead; unconventional advice, but you won't go wrong with it.