You're trying to SHRINK (as in decrease) MORE THAN (as in increase) 120 MB?
I read that as...you're trying to make your primary partition smaller than 120 megabytes? ... not sure why you'd want to...
But when I"ve needed to resize NTFS partitions to dual boot linux, I usually just grab a systemrescuecd , boot it, run: startx & then open the gparted application
YOu can shrink & grow NTFS/fat/ext partitions there & it works great...though I've never tred it on a Vista machine.