I've been reading good things about GParted (
GParted Live CD /
Parted Magic Live CD).
I'm going to attempt to resize my drive down 100GB with it so I can install Windows / programs into a separate partition. I hope it doesn't fail, because I have about 600GB of videos on the drive (I don't have enough free space on other drives to back them up)

.
Vista's resize will only let me shrink it by 3GB, I believe this is down to the fact that it'll only let me resize until it hits system files which it won't move.