Found myself looking for this info couple of times and hence the blog

Want to resize vista partition and give more space to your ubuntu. Here are some pointers. Have tried all and found gparted the best. Leave it up to you to pick ur choice:
A link to HOWTO resize your vista using an application bundled with your OS.

ELSE:
Buy Parition Magic.

ELSE:
Gparted [HOWTO].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.

Good luck.