Copied right from Microsoft's KB, here is how to extend a partition/disk.

• At a command prompt, type diskpart.exe.
• Type list volume to display the existing volumes on the computer.
• Type Select volume volume number where volume number is number of the volume that you want to extend.
• Type extend [size=n] [disk=n] [noerr]. The following describes the parameters:
  • size=n
    • The space, in megabytes (MB), to add to the current partition. If you do not specify a size, the disk is extended to use all the next contiguous unallocated space.
  • disk=n
    • The dynamic disk on which to extend the volume. Space equal to size=n is allocated on the disk. If no disk is specified, the volume is extended on the current disk.
  • noerr
    • For scripting only. When an error is thrown, this parameter specifies that Diskpart continue to process commands as if the error did not occur. Without the noerr parameter, an error causes Diskpart to exit with an error code.
• Type exit to exit Diskpart.exe.

When the extend command is complete, you should receive a message that states that Diskpart successfully extended the volume. The new space should be added to the existing drive while maintaining the data on the volume.

The original KB article can be found here: http://support.microsoft.com/kb/325590