Microsoft Windows 2008 R2 standard edition includes a license for a virtual machine, so when you purchase this you can load a system as a HyperV server, then load a single virtual machine under that.
Further Windows 2008 virtual machines require an additional license to be purchased. If you want to run a number of virtual machines, the enterprise version may be most cost effective as it lets you run 4 virtual machines, instead of having to buy a standard + 3 additional standards.
- Windows Server 2008 R2 Standard