The latest problem is the following:
Before creating a new VM, I try to delete the useless one and its traces, including what's left in Library.Failed to create the hard disk storage /Users/dianedvacca/VirtualBox VMs/WindowsXP/WindowsXP.vdi.
Cannot register the hard disk '/Users/dianedvacca/VirtualBox VMs/WindowsXP/WindowsXP.vdi' {eb207370-d51a-43a9-aa9e-53946662ce96} because a hard disk '/Users/dianedvacca/VirtualBox VMs/WindowsXP/WindowsXP.vdi' with UUID {84099327-d2b6-48bd-b1b9-bb84cb0010a4} already exists.
Result Code: NS_ERROR_INVALID_ARG (0x80070057)
Component: VirtualBox
Interface: IVirtualBox {3b2f08eb-b810-4715-bee0-bb06b9880ad2}