In that case if all you want to do is rescue your files I would download the much smaller ISO for Hiren’s BootCD 15.1
and write it to flash stick using this tutorial: Hiren's BootCD From USB Flash Drive (Pen Drive) » www.hiren.info.
Then boot the stick, choose XP, Explore Windows 7 to find your files and drag them to your external HD.
What is it exactly you're trying to do with Windows 7, repair it, run Recovery, or reinstall it? Are you following the steps in Troubleshooting Windows 7 Failure to Start
? Hirens can be used for several of those steps but you'll need a Windows 7 DVD or Repair CD to fully repair it or reinstall.