There are a number of commercial software utilities that can manage your partitions, like Partition Magic now owned by Symantec. But Partition Magic cannot work with Windows Vista.
However, Windows Vista includes a built-in functionality in Disk Management to shrink and expand partitions. No more 3rd party utilities needed! Open the control panel, switch to classic view and then click Administrative tools, computer management and then Disk management.

Create a partition

First of all, you should know that after you used your computer with a single partition on its hard drive, the files will become fragmented, and you may not resize it properly, so before everything else, be sure to defragment your drive. Now, backup your data, "just in case", and let's start!

• Click on the Start menu
• Right click on My Computer and click on Manage
• If you get a User Account Control dialog here, simply click Continue
• In the left pane, choose the Storage category and open Disk Management. What you will find   here are the partitions on your hard drive
• Right click on the partition that you wish to change.
• Choose Shrink Volume to shrink the selected partition
• Enter the new size for the partition, and click Shrink
• Now, you will notice that the partition is smaller, and some free space is available to create a  new one; right click on the "free space" area.
• Choose "New Simple Volume" from the menu that appears, then click Next.

By default, Windows Vista will use all the free available space to create the new partition. Change the size if you need, or simply click Next if you agree with the default size.

Now that your new partition is almost created, you have to baptize it, so choose a drive letter and click Next.

At last, you should change the file system settings and format your new drive. In most cases, the NTFS file system using the default allocation unit size and a Quick Format should be fine. If you want a nick name for your new partition, baptize it again, using a suggestive name, such as "Documents", for example. Now, you new partition, will be also known as "Documents". Click Next. and you're done!

Shrink a Partition

In the Disk Management screen, just right-click on the partition that you want to shrink, and select "Shrink Volume" from the menu.
In the Shrink dialog, you will want to enter the amount you want to shrink by, not the new size. For example, if you want to shrink your 50 gb partition by roughly 10gb so that it will now be roughly 40gb, enter 10000 into the box.

Extend a Partition

On this screen, you can specify the amount that you want to increase the partition by. In this case, I'm going to extend it back to the roughly 50GB size that it was before.

Note that the extend partition feature only works with contiguous space.