There are a few things you can do, but the easiest in my experience would be to boot from your install cd and re-install, setting the correct partition size then and there.
If you are entirely opposed to that, you can go into your control panel in Vista, search for the "create and format hard disk partitions," select it, shrink your Windows volume by however much space you want for another partition, name it whatever you'd like and use it as a partition that is accessible for both operating systems.
I vastly prefer the first method in my experience of installing Ubuntu many, many times
