Running Microsoft Exchange Server 2003 on Microsoft Virtual Server is another way of leveraging the power of virtualization to reduce hardware and support costs for your enterprise. But there are some important limitations you need to pay attention to if you plan on doing this.
First, your underlying machine must be running Virtual Server 2003 R2 and your virtual machines must be running Exchange Server 2003 Service Pack 2 or later. And of course you need to install Virtual Server 2003 R2 Virtual Machine Additions and use the Microsoft Virtual Machine PCI SCSI Controller driver as your SCSI driver for your VM. Finally, using Undo Disks is a no-no and you can't cluster together Exchange servers.
Apart from these limitations and requirements though, this approach is a great one to use if you're planning on server consolidation to reduce costs.