Win7 RC Guest doesn't detect on-board serial ports

[Frank M]

I am unable to get my Windows 7 RC Guest to recognize any serial port. I have a Vista 32-bit host with a functional COM3 port. The IRQ and I/O Port are standard for COM3, and it is on-board (no USB-to-serial adapter). In the VirtualBox setup, I have one serial port enabled, Port Number = COM3, Port Mode = Host Device, Port/FilePath = COM3. Windows 7 RC boots just fine, so that tells me that VirtualBox was able to open the port, but in Device Manager, the COM port is just not recognized. The new hardware detection wizard doesn't detect it, and if I add it manually, it is flagged as not functioning, even after a restart of the guest. I've also enabled and disabled FIFO buffers on both the host and the guest with no luck. (Taken from ​http://forums.virtualbox.org/viewtopic.php?f=2&t=18886)