Note to those of you trying to extend your Windows Server 2003 .vhd:
The trick is to use WindowsXP:
Windows Server will not allow you to extend the volume of a basic disk -- it has to be a dynamic disk in order to be extend -- but WindowsXP does not have this restriction.
So after you've created your new .vhd using the Resizer, attach that file as a secondary (or tertiary, etc.) hard drive to an XP VM, then use handyd's recipe above. Then you can shut your XP machine down, remove the vhd from it, and boot your original Windows Server 2003 from that .vhd.