Download Windows 7 SP1 .iso | Unofficial Windows Reinstallation and Related Guides by Philip Yip.

If you are trying to perform downgrade rights then you need installation media with HP system locked preinstallation as no Windows 7 key is provided for downgrade rights. I have these for Dell systems listed here http://dellwindowsreinstallationguide.com/

dellwindowsreinstallationguide