As so many of you emailed me directly after previous entries who found it difficult to get hold of the Virtual Machine additions hotfix from KB 900076 (which is still unfortunately out of date), I'm relieved to say the version of the virtual machine additions released with Virtual Server 2005 R2 (13.552) are available for public download. No more phoning PSS for thar hotfix :-)
You should install these additions in each Virtual Machine guest you have, especially if the guests are running Windows Server 2003 Service Pack 1. This will give you a significant performance boost.
If you are running WS2003 SP1 guests under Virtual PC 2004 SP1, you should get similar performance boost but this is not supported or tested -further, if I interpret the licensing correctly, to do this you must have a licenced copy of Virtual Server 2005 for the host running Virtual PC.
Download the additions here