If you are trying to expand a system partition on windows 2003 Server, you are going to find, that there is no on-board solution to achieve this.

Even Diskpart will tell you, that it will not support this function.

But there is help from DELL
They created a simple and small command line utility, which can do exactly that.
Download the tool here:

http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverid=R64398

First of all, you will have to add more disk space to the System you are trying to expand.

In case of a virtual machine, simply edit the disk size in your virtualization management software.

In case of a RAID Disk, you could add more disks to the RAID, or replace the existing disks with larger ones, one by one. (e.g. replace one, wait for rebuild, replace the next one and so on.)

In the end, use your RAID Manager software to expand the RAID Partition .

Once this is done, fire up your server and open Computer Management. Check Disk Manager to see if the system can see the additional space (if you can’t see it, use the “rescan disks” command before you look further)

If you can see the free space, you are ready for the expand

Open a command prompt and run the extpart.exe command.

Enter your System partitions drive letter when asked (e.g. c:)

Then enter the amount by which you want to extend the partition in MB

So if you have a 15GB partition and 10GB additional space, enter 10240 (10*1024MB)

Hit enter and your partition will be extended.

This will work for non-system partitions too by the way.