Here's instructions for the Microsoft Windows 7 website
You must be logged on as an administrator to perform these steps.
Yes. It's possible in this version of Windows to repartition your hard disk by using the Shrink feature in Disk Management. You can shrink an existing partition or volume to create unallocated disk space, from which you can create a new partition or volume. (Often, the terms partition and volume are used interchangeably.)
Open Computer Management by clicking the Start button , clicking Control Panel, clicking System and Maintenance, clicking Administrative Tools, and then double-clicking Computer Management.* If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
In the Navigation pane, under Storage, click Disk Management.
Right-click the volume you want to shrink, and then click Shrink Volume.
Follow the instructions on your screen.
http://windows.microsoft.com/en-US/windows-vista/Can-I-repartition-my-hard-disk