Shrinking to the right means you are taking away from the file sytem to the end of the disk. Shrinking to the left means you are taking away from the file system to the beginning of the disk.
The beginning of the disk contains your file table that describes how the partitions are laid out. You cannot shrink to the left because it would throw the file table out of whack, and Windows will not let you do that. It also screws with the Master Boot Record, which is required to boot into Windows or any other operating system. When you move the beginning of a partition(your 'right' partition), it throws the File Table indexes of all following files off by however much you expanded to the left. These must be corrected in order to use that data again. Windows DM doesn't offer this functionality, so it doesn't allow you to do it. Not the most technical explaination but it's my basic understanding.
The easiest tool I have found is GParted, which is a Linux program. It allows you to shrink to the left. Be aware that it will scramble your Master Boot record, and you will not be able to boot into windows until you repair it. The basic steps I have used to do what you are describing are:
1) BACKUP ALL DATA!!!
2) Boot into a GParted Live CD
3) RePartition the drive to your liking from GParted
4) Reboot into Windows Recovery using your original installation CD
5) Pick repair mode, since oyu do not want to install windows again
6) Scan for problems, and let windows fix them for you
Windows is pretty good about repairing the MBR in cases such as this. I have done this 4 times and it has worked perfectly each time. Do not attempt this without a Windows installation disc, or a full backup of your data. If something does go wrong, you will trash the filesystem and getting your data back could be difficult/impossible.