I have run out of space on my WinXP virtual machine, which I only gave 10 GB space for when I created it. Is there an easy way to increase it to, say, 20 GB? I can't see any obvious option in VirtualBox settings.

edit: the suggestion below gives this error

wim@wim-ubuntu:/media/data/winxp_vm$ VBoxManage modifyhd wim.vdi --resize 20000
VBoxManage: error: Cannot register the hard disk '/media/data/winxp_vm/wim.vdi' {46284957-2c09-4e70-8a49-bfbe0f7f681d} because a hard disk '/home/wim/VirtualBox VMs/winxp_vm/wim.vdi' with UUID {46284957-2c09-4e70-8a49-bfbe0f7f681d} already exists
VBoxManage: error: Details: code NS_ERROR_INVALID_ARG (0x80070057), component VirtualBox, interface IVirtualBox, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType, AccessMode_ReadWrite, fForceNewUuidOnOpen, pMedium.asOutParam())" at line 210 of file VBoxManageDisk.cpp

edit2: removing the .vdi from VirtualBox before calling VBoxManage command, then adding it back in, was successful. But now I can't boot the virtual machine, I get this worrying screen:

By the way, it says FATAL: Could not read from the boot medium! System halted.

edit3: The vdi must be reattached to the VM after VBoxManage command. Further, the partition will need to be resized from WITHIN windows, because you will have this empty space:

I was able to resize the partition easily using a bit of freeware called EASEUS Partition Master 9.1.0 Home Edition.