The "6421A" course is a Microsoft Official Learning Product, which is sold to training centers for a price. The training centers can opt to install the labs on their own computers, or to pay a fee to have the labs hosted in a virtual environment that can
be accessed remotely. However, this service is not available directly to you; only the training centers can arrange it.
On the other hand, if you want to experiment with Windows Server without installing it on your own computer, you may wish to try out the Technet Virtual Labs:
http://technet.microsoft.com/en-us/virtuallabs/default.aspx