If you want a fairly easy way to install a Linux distro on a Windows partition without repartitioning or anything, you should check out some of the
coLinux distributions such as
Topologi Linux. Topologi is based on Slackware and it'll run as a process under the Windows kernel. Instead of making a new partition, it creates a really big (or small, depending on how much space you specify in the install) file that acts as a virtual partition for the system. Speaking from personal experience, I've used Topologi on my own computer and was very pleased with it. Networking wasn't too terrible to get working as long as you follow instructions, and configuring X isn't so bad either (just make sure your firewall doesn't block VNC). Also, if Slackware's not your thing, then there are other distros based on Knoppix, Gentoo, Debian, Fedora, and perhaps others as well. Hope that helps!
