To resize a partition with Windows Vista, follow these steps:
Be sure to back up any valuable information, because there is a chance that data can be lost when dealing with partitions.
1) Click on the Start menu
2) Right click on Computer and click on Manage
3) You may get a User Account Control dialog here; just click Continue
4) In the left pane, open up the Storage category and click on Disk Management
5) Here, you will find your partitions for your disks. Right click on the partition you’d like to modify.
6) Click on Extend Volume or Shrink Volume to extend or shrink the selected partition.
If this doesn’t work for you, try this page.