I have three Macs I want to install Windows 7 Home Premium on. Never had Windows install on any of these Macs. This will be the first time. Which Windows 7 do I purchase? 1. Home Premium Family Pack Upgrade 2. Home Premium single use The reason why I ask is...Isn't single use Windows 7 only allow one computer installation? However, if I get the Upgrade, then I have licenses for 3 computers, but I have to have a previous version of Windows installed for it to work...no? I'm so confused!