I could be wrong, but I'm pretty sure you can just create a new partition from within windows. Google will help you out on that.
How fast is your computer? A better option is probably to try Linux in a virtual machine first. I run Ubuntu on my XP desktop using the free VMWare server edition. It works great and doesn't require you to reboot every time you want to switch OS's.
http://www.vmware.com/products/server/
I've used a couple of different Linux distro's and by far my favourite is Ubuntu and definitely what I would suggest for someone who just wants to get a small taste of linux. It is the "easiest" linux distribution I've worked with. You should be able to go here
www.ubuntu.com/getubuntu/ if you want to download it.