Resize partition of Windows 7 running on VirtualBox with dynamically allocated storage

You can only resize disks if they are not part of a VM that uses snapshots.

If you are using snapshots, you can easily clone the state to a new VM and hard disk, and resize that one. VirtualBox 4 now supports cloning from the GUI, so it's super easy.

VBoxManage - modifyhd

Here's a helpful walk through of the whole process, Resize and Expand a Virtualbox Hard Drive and Media in 4 Steps