Do a google search for "gparted live cd". The first one that came up on my PC was
http://gparted.sourceforge.net/livecd.php
It states, "GParted Live is a small bootable GNU/Linux distribution for x86 based computers. It enables you to use all the features of the latest versions of the GParted application."
There are instructions for creating a bootable CD or bootable USB stick. You can use the partition editor to shrink the OS down to the size you want. After that you can either create a new partition from the unused space with gparted or you can boot into Windows and do it from there. Probably best to do it through Windows. Grab a book. It may take a while.