I work with Virtual PC and Virtual Server all the time, but am continually astonished by some of the problems I see people having with Virtual machine technology. Here's one of the most common problems I see, and it frequently seems to catch out so many people even internally at Microsoft. Often, Virtual Machines are built some time ago and put on the shelf ready for use at a future point. Good examples are for things like pre-canned demos or just a scratch dev/test machine for a multitude of reasons. While people (mostly at least) remember to patch their machines through Windows Update, most of the time, they forget to update the Virtual Machine additions.
The correct version of the VM Additions is critical to get the maximum performance possible. As an example, I know of many people who are using Virtual Machines originally built from the base Virtual PC 2004 product. Obviously, SP1 for VPC came out some time ago now, but they continue to use the original additions from the RTM product. Similarly, any VMs built using the Virtual Server 2005 RTM additions installed and now being used under SP1 Beta should similarly be updated with the VS2005 SP1 Beta version of the additions. If you have VMs that are compatible between Virtual PC and Virtual Server (for instance, you don't have SCSI disks configured), then the general rule is to always use the latest version of the additions available.
Today, the latest build of the additions is from the SP1 Beta for Virtual Server 2005 (Build 13.518). You can find out the version of the additions installed from add/remove programs in the Control Panel within the guest. Select the virtual machine additions, and click the support hyperlink.