I bet this has been asked a million times but I'm so confused, I've built a new PC and it has no OS nothing, I bought my OS from a guy on EBay who sold it for $70 he gave me the license key but no DVD, So I don't know how to install it? I'm pretty sure someone told me to use a different computer and install the OS onto the USB and put it on the new PC but my only computer at the moment is a Mac what do I do?