VirtualBox version: 1.5.6
Host OS: Windows XP
Guest OS: Ubuntu Gutsy, 7.10
Machine type: Lenovo T60p laptop (mounted on a docking station)
If your windows com port is com1, configure virtual box as shown in the figure:
Open the windows device manager (can be opened by right click "My Computer" in Explorer->Properties->Hardware tab->Device Manager) and check the configuration for the com port your specified for your guest virtual machine. You will have to set the configuration parameters same as the client that you use in the Linux guest.
Setting similar properties for both enabled me to use serial port flawlessly