I suggest Linux Mint 6.0 (Felicia)
It's based off Ubuntu, and it's easy to use for beginner like you (and me :-D). It's fast, stable, and looks very sexy. It's defiantly one of my favorite Operating Systems of all time.
Here's a link to it: http://www.linuxmint.com/download.php
It even has the option of creating a Dual Boot with your PC.
My PC has Window's Vista Basic & Linux Mint 6.0 on it.
I use Linux for doing basic things like web browsing, writing notes, doing things with pictures, playing music.
And I use Window's for everything else: Gaming (Call Of Duty 4 :-D), Making and editing Videos, and everything thing else and programs that aren't compatible with Linux.
You can also get "Wine" for Linux. Wine is a program that lets you install and use many of the basic programs and games you'd use for Windows. http://www.winehq.org/ (Wine can be installed on other OS like Mac/Apple, FreeBSD, Solaris, etc...)
And here a list of some of the compatible games and programs that run on Wine: http://appdb.winehq.org/
I hope you enjoy Linux. I highly suggest getting Linux Mint 6.0 (Felicia)
EDIT: Oh, and Mint has the option of running the OS from a CD. It's called a "Live CD". You just download the .iso image for Mint at there website, and burn the .iso to a CD. Then you just leave the CD in your CD driver and start your PC up! Then you can start using Mint without installing it and it harming/damaging the OS that you already have on your PC.
And if you need any help with Linux (Especially the Mint versions) I would talk to "LUg" up there ^. I can see he's already answered this question, and he's a genius when it comes to everything Linux-wise. He's helped me it the recent past with Linux, and I'm very appreciative for that! :-D