You can download a Windows 7 Pro ISO file and then burn it to a DVD on another computer, then boot the HP with the burned disk.
http://www.techverse.net/download-windows-7-iso-x86-x64-microsofts-official-servers/
This is probably a better source for Windows installation discs:
http://www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/
Post back if you don't know what to do with an ISO file once it is downloaded.