Furthermore, in the case of upgrade media (as jt mentioned earlier), even if the previous owner is still using the key with an upgrade, technically its them who is breaking the law, not you, as the license usually goes with the original version, not the upgraded version. For example, my PC originally came with Windows Vista. I purchased the Windows 7 Upgrade and upgraded it from Vista to 7. Now, lets pretend that my copy of Vista is actually a retail copy (its actually OEM), which would give me the right to sell it seperately from the PC, and I decided to do that, since I don't need it, now that I'm running 7. Two different "copies", with two different Product Keys even. However, my copy of Windows 7 is simply an upgrade for my old copy of Vista. The two copies are still under the same license - the original Vista license. By selling that copy of Vista to someone else, I would be giving up my right to use my Windows 7 upgrade.