disk management utility
I'm at work right now, so I'm looking under XP:
My Computer -> Manage -> Disk Management
I *think* it's the same with Vista. (I'm terrible at this without the OS is question sitting right in front of me).
If you're just doing a re-install, I don't know if you'd necessarily need to mess with the partitions...but then, of course, I don't know your machine.
I did a clean re-install of Vista a few days ago and when it asked me where I wanted to install I just indicated the volume that Vista already called home. To my joy, I found the process quicker and easier than for any other Windows OS I've worked with.