I don't dual boot, I run Windows Vista. Yes I know, a lot of people hate it (I was one of them), until I actually used it.

I've ran Redhat, Ubuntu, Slackware, Debian, Fedora, OpenBSD, and FreeBSD. By far the easiest and user friendly, IMO is Ubuntu.
Before you go dual boot, download VMware Server, it's free and you can run multiple OS on top of your host OS.