so the upgrade is only for the previous versions of windows? i thought i could run a clean install on my new hard drive
Well, technically with the upgrade version, you have to have a previous copy of XP or Vista installed and it has to be activated. However, there are work arounds so that you can do a clean install of Windows 7 without having to install XP or Vista first.
However, if you purchase the upgrade version of Windows 7 and you do NOT have a previous, legit copy of XP or Vista, then you are violating the license agreement because when you purchase the Upgrade version, you are stating that you already have a previous license from a previous copy of XP or Vista. So, in effect what you are doing is you are transferring the XP or Vista license to Windows 7. This makes the copy of XP or Vista no longer legitimate and means you are not allowed to install it on another machine. The key for XP or Vista is invalidated so it will no longer activate, or at least, that is what's supposed to happen.
So, in simple English, if you purchase the upgrade version of Windows 7 and you do not have a legit copy of XP or Vista, you are illegally installing and using Windows 7.