There is always an unavoidable risk combined with partitioning operations. That is why all manuals remind you of making a full backup before proceeding.
When it comes to the system partition, i.e. the one that your OS resides upon, the risk is even higher because, if things go wrong, you do not only lose data which may easily be restored from a backup, you may also have to face reinstalling your operating system.
In order to create a new partition there must be empty space, either at the beginning or the end of the existing partition. So in order to create a new partition, the partition manager must move a lot of the existing data in order to create the blank space. You can facilitate this by using a good defragmenting tool prior to doing the partitioning, but on Windows system partitions there are a lot of reserved protected files that may not be moved by defragmenters. This, obviously, increases the risks somewhat.
I recommend the small Parted Magic live CD:
www.partedmagic.comIt is free, has a GUI that makes it easy to use for Windows users, and applies GParted which is arguably the best free partitioning tool around.
Here's a step by step illustrated guide on using it for exactly your purpose:
http://partedmagic.com/documentation/119-using-gparted.html
