Information on Microsoft's "Virtual Machine" technology seems to be appearing now, we've got some builds internally and they look really good.

The ability to base a new VM on another and only store the changes is really good, saves loads of disk space!  You can also drag/drop and use the clipboard between VMs which is handy

Microsoft Virtual PC information here:

http://www.microsoft.com/windowsxp/virtualpc/

Microsoft Virtual Server Information and Beta details here:

http://www.microsoft.com/windowsserver2003/evaluation/trial/virtualserver.mspx