Extending Basic Disks on windows server 2003 using DISKPART.exe does not work.
To extend the basic disk with 3rd party tool using EXTPART. ExtPart is a small tool developed by dell Industries and which helps to extend disk on windows 2003 servers as follows
Usage:

extpart [volume size]

volume - the volume to extend. e.g. f:, g:, etc (only basic volumes)

size - the size in megabytes by which to extend the volume.

Example:

To extend the g: volume by 1 GB (1024 MB) use the following command:

extpart g: 1024

Download Link: http://www.dell.com/support/drivers/in/en/19/driverdetails?driverid=R64398