Hi:
Here is a link to all of the W7 ISO downloads.
Select the SP1 Home Premium 64 bit Media Refresh ISO file (X17-58997).
Use this tool to transfer the file to a 4 GB USB flash drive or DVD. The tool will make the media bootable as well.
http://www.microsoftstore.com/store/msusa/html/pbPage.Help_Win7_usbdvd_dwnTool
The product key is only good for the version of windows listed on it.