Installing Windows 7 clean from an External Hard Drive

Earlier today my laptop got hit with a nasty virus making everything inaccessible luckily, most of the stuff that matters to me is on my external. I tried installing Windows XP and the disc wouldn't install properly, it formatted my operating system but couldn't find like 10 files, so I called it quits. I know my laptop can handle Windows 7, I just chose to use XP because it had less bugs when I got it. I got a friend to forward me a copy of Windows 7 and it's currently on the external hard drive and I've been searching around for a way to install it from the hard drive. I've found out a myriad of ways to install from a flash drive, but I have no idea if it's the same process with an actual hard drive. In addition to those methods, I've also found ways to install it from within the computer I wish to install it in. The only problem is those require Windows which I don't have and can't get anymore.

So, I have a clean laptop with no operating system. How do I install Windows 7 from my external hard drive?