Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
One of the more advanced options for resizing your Windows Vista partition is to use the GParted Live CD, a bootable linux CD that takes you straight into GParted, the great linux utility for managing partitions. The problem is that if you resize your boot/system partition, you will be completely unable to boot without repairing windows.
First make sure that you have a bootable Windows Vista installation DVD, as you will be unable to use your computer if you don't. Next, download the GParted Live CD and burn it to cd.
Boot off the GParted cd, and you will see your hard drives in the drop-down list. The first drive is typically your boot drive, but you can check the Flags column to make sure.