Just a note as I’ve had to extend a fixed size VHD. I used http://code.msdn.microsoft.com/vhdtool to expand the actual VHD file. This extending the partition, but did not extend the volume. To complete the process, I needed to boot the VHD and extend the volume (C:).
As I wanted to extend my boot volume (C:), I could not extend onto this using Disk Management (from Computer Management). Instead I had to use the Diskpart tool (access via the command line when ran as Administrator).
I then ran the following commands:
diskpart (this takes you to the diskpart prompt).
select disk 2
select volume 3
Once complete, my fixed size VHD was expanded to 60Gb