The most recent version of Linux Ubuntu, Hardy Heron 8.04, features a WUBI installer (Windows-based Ubuntu Installer) that allows you to install Ubuntu from windows onto your windows partition, allowing you to boot both operating systems without repartitioning your hard drive.
Linux Journal Gadget Guy, Shawn Powers, demonstrates installing Ubuntu on a machine running Windows with the Wubi installer in this 4 minute video:
Wubi makes it much easier for Windows users who want to try Linux without having to repartition their hard drive.
I've been using Ubuntu Hardy Heron 8.04 lately, and it's the most solid Ubuntu I've tried to date. I didn't use the WUBI installer, I installed it to it's own partition, but WUBI is a good option Windows users to start with.
I think some Windows users might prefer a more polished Ubuntu derivative, like Linux Mint, which has all the multimedia codecs already installed for you, providing a more immediate windows-like experience. But since Ubuntu 8.04 was only just released, it will be a bit of time before Linux Mint and others come out with their versions, but we should be seeing them appearing soon, hopefully with the WUBI installer included.
You can read more about WUBI here:
WUBI FAQ (Frequently Asked Questions, with screenshots)