Regular versions of Windows are licensed per machine and it doesn't matter if it's a physical PC or a VM. It doesn't matter if the software comes on a physical disk or as an ISO.
Windows 7 Enterprise was an exception and I believe it applies to Windows 8 Enterprise too:
Software Assurance customers using Windows 7 Enterprise may install up to four virtualized instances of Windows 7 Enterprise or Ultimate on the machine which has Windows 7 Enterprise installed. Other versions of Windows 7 do not allow for virtualized instances of Windows 7 to run without a separate license. (source)
Another exception is the DreamSpark license which applies to Windows copies available for free for college students. Those can be installed only on virtual machines or as an upgrade for physical PCs that already have licensed version of Windows installed.