Someone simply threw that on a page for a quick reference without getting some facts apparently. This has to do with what was mentioned before about unmovable system files seen at the end of the boot/system primary. For someone new the following should be heeded.
"MOVING OR DELETING OCCUPIED (SYSTEM) FILES
The (system) files which are stored on the last occupied sectors of the partition are deleted or moved safely by (temporarily) disabling the specific process which holds the file (visit the page disk cleanup (Vista) / disk cleanup (XP) for more information about disabling the mentioned features). The defragmentation tool VOPT (download:
www.vopt.com) can be used to tell which files are stored on the last occupied sector. These are the files to move or delete to be able to shrink the partition."
http://www.optimizingpc.com/miscellaneous/resize_windowspartition.html
The best part is what Cromewell already suggested before namely selecting a 3rd party tool and getting familiar with that since those will work without Windows being active.
The free partitioning tool GParted
When the Windows Disk Management tool doesn't result into the preferred resized partition, then it is time to apply more advanced partitioning tools. In case of free software, the tool GParted is most used at this moment. GParted is integrated into multiple bootable Linux distributions, like the GParted Live CD (download:
http://gparted.sourceforge.net/livecd.php). After starting the computer from the CD, the Windows operating system is not active anymore. Because the files which were previously occupied by a process are now free, they can be moved by the partitioning tool."
Unless you are experienced you wouldn't be going around deleting system files or disabling certain processes in order to see the DM shrink the boot volume when there are effective tools that run independently. The continued advice for sandcracker21 would be to review the documentation on GParted being a free drive tool readily available for the cost of a blank cd-r seen at
http://gparted.sourceforge.net/larry/generalities/gparted.htm