This seems to be something that my Windows Vista book did not mention about disk management in comparison with the past versions.  Windows Vista/7/2008 Server have an improved Disk Management feature in that it allows you to shrink basic partitions.  Whereas in the past we have had to use 3rd party utilities (such as gparted or partition magic) to resize drives, Windows has the option to shrink the partition size.  Simply open up disk management and right click on the partition you wish to shrink and select “Shrink”. 

Windows will calculate exactly how much it can shrink so that you can use the new unallocated space to make additional partitions.  Limitations to this of course depend on where the data is currently stored on the disk.  If it is scattered, you may be able to claim more by defragmenting and moving your data towards the first sectors of the disk.