This is easy.
I'm a moron-level user (that's below totally confused
and I have done this.
First do your housework, clean out the Recycle bin, temp files, etc.
Defrag.
Backup.
Download BurnCDCC from here :
http://www.snapfiles.com/get/burncdcc.htmlPlace it where you can quickly access it.
Download GParted from here :
http://gparted.sourceforge.net/livecd.phpLeave it unopened on your desktop.
Open BurnCDCC and tell it to burn GParted.
A few minutes later you will have a very effective Partitioning Utility that is bootable.
With Vista running, open the CD tray and insert GParted.
Shut down and reboot.
GParted will start and hold your hand through the partitioning process.
Leave plenty of headroom for Vista and you won't have any problems.