I cannot emphasize this enough:
DO NOT USE WINDOWS PARTITIONING TOOLS ON YOUR HARD DISK!
Your disk almost certainly uses a hybrid MBR, which is the traditional Apple way of installing Windows on an OS X disk. The trouble is that a hybrid MBR defines some of your partitions twice, and most Windows tools will only modify one of those two definitions, leading to a dangerous inconsistency in your partition table. The result, at best, is a need for extra steps to repair the damage. At worst, you could lose significant amounts of data.
Broadly speaking, you must use GPT-aware tools to do the job. Apple's Disk Utility can resize HFS+ partitions, but I'm not sure offhand if it can handle NTFS partitions. (My guess is that it can't.) Thus, you may need to resort to something like GParted in Linux (from a Linux emergency disc like Parted Magic or System Rescue CD). Unfortunately, this could result in a need to re-create your hybrid MBR, since GParted converts them into legal GPT data structures. (Hybrid MBRs break several important GPT rules.) Furthermore, if you change the start point of the NTFS volume, Windows is likely to stop booting and will require repair by a Windows installation/repair disc. It's conceivable that some third-party OS X tool can do the job, too, but I'm not very familiar with what's available in this category.
It may be simpler and safer to plan on backing up Windows to the external disk you mention, delete the Windows partition (in OS X's Disk Utility), resize the OS X partition, create a new partition for Windows, and then restore your Windows installation to the new partition. When you create the new Windows partition, do so in OS X's Disk Utility by creating a FAT partition. This will cause Disk Utility to set up the hybrid MBR data structures, so you should be able to restore your backup using Windows utilities. There are of course things that can go wrong with such a procedure, but it stands a good chance of succeeding.
No way to do this is risk-free, so you should back up your OS X partition, too.