You will need to contact either Dell or Microsoft to figure out your license for win7 (I'd say call up dell first)
an OEM lincense, which is what dell typically uses, limits your windows installation only to your computer. However, the way it keeps track of whether this is your machine or not is by remembering the system configuration of your hardware. So, when you changed the hard drive the system configuration changed, which is why it is not letting you install windows on it.
The computer originally had Vista. I installed a new hard drive so the operating system is now gone. So why can't I just install a new hard drive and install Windows 7 on it? I don't see why I can't. There are no errors but it seems the hard drive is not being told to load. The old hard drive had a password. Does this matter?