Also, don't forget the remote console abilities built in to VirtualBox. So if you are running any machines (Linux or Windows) as a VBox guest on any host (Windows or Linux) you can rdp to that guest from a Windows machine if you enable The Guests Remote Display

Settings-> Display -> Remote Display

Keep in mind, the default here is port 3389, which is the standard port for RDP. If you have multiple Guests on a single host, you'll need to assign each their own port. (Also, remember if you want to rdp to the host, it will also be at that 3389 port.)

Personally, I simply increase the port number used by each guest as I set them up.

Host:3389 = The Hosts display
Host:3390 = Guest #1
Host:3391 = Guest #2
Host:3392 = Guest #3
etc