Increasing the size of a VHD

Increasing the size of a VHD file is done in two steps.
Step 1 is to increase the size of the file itself. That can be done with the VBoxManage utility found in the c:\Program Files\Oracle\VirtualBox directory.  Run the following:

VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB

Step 2 is change the partition to take up the extra space created in Step 1. That can be done with the DiskPart tool that is built into Windows. To do this,  Launch the virtual machine, go to a command prompt (Administrator Mode), and run:

diskpart

Get a list of the volumes mounted on the machine:

list volume

From the resulting list, get the number of the volume for the one you want to extend. Select the volume:

select volume x

Where x is the # you got from the list volume command. Verify you have selected the right volume:

detail volume

Extend the partition to take all available space:

extend

Shut down the virtual machine, disconnect the volume, and it should be ready to mount to the original virtual pc.