for VMware users here's a nice tip:
set this option in virtual machine configuration: serialN
where N is the serial port number and X is a positive integer that expresses a percentage of the default speed (30 seems a value that work good)
then select from port advanced configuration on guest the maximum speed available for the com port (128000), and select the same speed when connecting with windbg.
two similar methods that seem to be more reliable, consist in a driver to be loaded on the guest and a patch to be applied to the virtual machine software on the host
VMKD (http://www.nynaeve.net/?page_id=168) which works only on VMware
VirtualKD (http://virtualkd.sysprogs.org) which should works also on VirtualBox.