Okay, so you've decided to switch to Linux. You won't regret it!
If you want to keep Windows on your computer (which I recommend, in case you change your mind, or in case you find stuff you can only do with Windows), Windows is gonna need to share the disk with Linux. Then, each time your computer starts, it will let you choose between Windows and Linux.
You should first backup all your data. This is extremely important. We never know what may go wrong. No, really, I'm not kidding. Backup your data, get blank CDs, blank DVDs, another hard disk, whatever, but back up your work.
Then you should defragment your hard disk. Right now, your data is scattered all around (Windows doesn't manage this very well, Linux does much better). After defragmenting (this can take a few hours depending on the size of your disk), all the data will have been gathered at the beginning of your hard drive (think of it as a long linear tape, with a beginning and an end). If your data is gathered there, Linux won't have any problem splitting (virtually, of course :) ) your hard disk in two, and install itself on the second part.
Okay, you're all set, now all you need to do is choose which Linux distribution you want. Linux comes in a lot of different "flavors", built by different organizations or companies, with different sets of software, differents look & feel, etc. They're all Linux, but with different "interpretations". This is not an obvious choice, and I can't really help you there (actually, lots of people like to argue about their distribution being the best). However, here are the four most popular distributions, ranked by distrowatch.com :
When you have chosen a distribution, download the CD(s) image(s). Then, burn it (or them) onto blank CD(s). If you're not sure how to do this or you'd rather pay a few bucks and get CDs delivered to you, you can try EasyLinuxCDs.com, they have a pretty good reputation. Now insert the first one in your CD drive, and reboot. Your PC should use the CD to boot ; if it doesn't, your BIOS configuration is faulty (see the "Try Linux" section).
After that, just follow the instructions on the screen : they're pretty simple and straightforward most of the time !
Have a nice time with Linux!