QEMU runs on a lot of platforms and can emulate also a lot of other platforms. It is, for example, used to run Windows 95 on my mobile (ARM based) game console. So this should be what you may want. If it can emulate the AMD64 instruction set is unknown to me right now, but I think that answer is only a search away.
Thanks for your answers here. I'm checking QEMU but I've problems with the GUI, I would like do without it, but I think it's impossible. So, I'm trying with the option with VNC Server which is included but try QEMU in VirtualBox cause another problem with the network. So, I will try directly on a FreeBSD on a computer. Thus, I hope problems with network will disappear.
If you don't want add comments, I put this thread as resolved. Sorry for the late answer, I was busy.