Old hard drive crashed, new hard drive. Trying to install windows using key on old computer

My hard drive just crashed and I have a replacement one. I tried to download the ISO file on microsofts website using the key I have but I get the error that the OS has been pre installed and I'm directed to contact Dell. I've been through every customer support line and it all leads to me being transferred to different departments. I just want to either use this key or get a replacement windows CD or something along those lines.