Recently I needed to resize the C drive on a Windows 2008 R2 virtual machine from its existing 10GB to 20GB.
I performed the following steps using VirtualBox v4.
1. Execute from the command prompt (Note the Virtual Machine was Powered Off)
VBoxManage modifyhd C:\VM_Test_Env\Servers\W2K8R2-04.vdi --resize 20480
2. Start the virtual machine.
3. In Disk Management – right click the C drive – select Extend Volume – follow the Wizard prompts.
You should end up with a larger C drive.