How do I extended a VHD C: drive size in Hyper V?

Shut down the VM. In Hyper-V Manager edit the VHD and expand it. Attach the VHD to a different VM and use Disk Management or diskpart.exe to expand the C partition (most versions of Windows don't allow you to expand the running boot volume). Dismount the now expand VHD from the other VM and reboot the original VM.

Ok great, I will give that a go. I have done the expand part before, but always found it comes back as a new drive, with no ability to extend the original C partition.

If your server is windows 2008 R2 you can (after you have stopped your VM) use disk management choose attach VHD, then right click the volume and choose extend volume and at the end choose detach VHD and your done.

Ronnypot, it is a 2008 R2. So are you saying I use the disk manager of the host to extend the size of the VM?