Technet Magazine have a great article on Microsoft Virtual server 2005. If you looking into what virtualization is all about have a read of that article and then head over to
here to get an evaluation version. Download or Order CD Here
Update: The link to the evaluation version has expired. Probably because Microsoft are now giving the software away for free. Microsoft Press Pass article here. They will also start to offer full Tech Support for Windows Virtual Server R2 aswell as Virtual Machine Additions for select Linux Distributions.
Update2:The VMAdditions will support the following disto's. You can get the beta Linux VMAdditions from http://connect.microsoft.com and look under the available programs once signed in.
-Red Hat Enterprise Linux 2.1 (update 6)
-Red Hat Enterprise Linux 3 (update 6)
-Red Hat Enterprise Linux 4
-SuSE Linux Enterprise Server 9
-Red Hat Linux 7.3
-Red Hat Linux 9.0
-SuSE Linux 9.2
-SuSE Linux 9.3
-SuSE Linux 10
Thanks to John Howard for the updated info.