This is probably the Nth page about this subject, so consider it a personal reminder.
How to allow direct access to the physical serial port to a virtual machine under Virtualbox (currently version 4.0.x) ? Edit VM's properties and flag as shown in the screenshot below:
[Info: "Dispositivo host" should translate into "Host device" in English.]
Settings shown above worked both for a MS and a Linux guest virtual machine.
Power up the VM and it will appear as COM1: under a MS OS or /dev/ttyS0 under your preferred Linux distro. For already installed Windows machines you'll have to fiddle with new hardware detection, and do a web search to find the most straightforward procedure.