I made an image with disk2vhd and unfortunately I chose VHDX instead of VHD. I didn't see any information about what to choose.
Anyway, now that file is all I have, the original PC is trashed. So I need to know how to run it as a Virtual Machine in Windows 7.
VirtualBox doesn't have an option to open VHDX and WM Converter doesn't have an option to open VHDX (or to convert to VHD).