If you get a "whitebox" OEM copy of Windows, you'll be fine. Microsoft calles these DSP or COEM versions. (Whatever those mean.) They are still OEM copies of Windows and are still tied to a specific set of hardware. But unline a Dell OEM copy of Windows, which can only be activated on Dell physical systems, a DSP version will activate on any hardware, but cannot be reactivated on different hardware if you move the copy of Windows to a different motherboard. To quote Tom above and what I read from a MS rep: "The DSP license is tied to the specific computer that it is installed on and the license lives and dies with the PC." From a legal standpoint, OEM / DSP /COEM versions I thing are supposed to be sold with "non-peripheral" hardware such as a motherboard. But how that ties in with virtual hardware, I'll leave to the legal types.
So if you purchase a separate OEM / COEM/ DSP copy of Windows (one that didn't come with your PC,) then you should be ok with using that in a virtual machine. But trying to use the WIndows that came with your PC won't work. Only if you're purchasing an unbranded PC (not a Dell or HP) and only if the seller/builder sells the system with a DSP / COEM version of Windows and only if the seller/builder does NOT activate that copy of Windows, then it could work. But any branded OEM version of WIndows won't work.
