We know that we get option of our hard disk partition during windows installation. But what if we want to repartition our hard disk after windows installation. Here is the method:

1. Login as administrator.

2. Navigate to:
Start -> right click My computer -> Manage

3. Computer management page will open

4. Select "Disk Management" under "storage"

5. Right-click the volume you want to shrink, and then click Shrink Volume.

Follow the instructions on your screen...and you are done.

Note: This applies only Windows Vista.
For Windows XP, use third party software like Magic Partition.