Per quanto riguarda Windows 7, da qui:
Licensing the Windows Desktop Operating System

For Windows operating system software licensed through retail (FPP) or preinstalled on a PC (OEM), Windows use rights are outlined in the Software License Terms that accompany the software. These license terms provide use rights to run Windows locally on the licensed device in a virtual operating system environment (OSE); however they do not provide use rights for accessing Windows running remotely in a virtual OSE from the licensed device, and are limited in other ways when compared to virtualization use rights provided with Windows Software Assurance, Windows Intune, and Windows VDA licenses acquired through Microsoft Volume Licensing. For example, neither FPP nor OEM licenses permit remote access to a Windows virtual machine (VM) running in a datacenter. For this, a license obtained through Microsoft Volume Licensing is required.
Quindi dovrebbe essere permesso. Per Vista credo sia lo stesso, per XP non credo che avessero previsto alcunché di specifico nella licenza, ma non vedo particolari ostacoli - una licenza OEM impone che tu esegua il software sul PC fisico a cui è legata, e il fatto che stia girando in una macchina virtuale non dovrebbe cambiare la questione.