Well, if you're not replacing Win8 with Win7, then you have a different problem than the one I described.
But here is something to consider:
1) You are trying to reinstall Windows in a HP computer with the same Windows software that came with the computer, then this is probably the source of the problem, because the HP Windows software was custom made for your machine with the drivers tailored to everything in that machine: including the hard drive.
Ergo, your windows installation won't work, because the hard drive drivers included with the HP WIndows are the wrong ones.
If this describes your situation, you will have to do what I did, because I had a similar problem when I replaced the original hard drive with an SSD. You will have to go buy a new copy of WIndows 7, that is called "SYSTEM BUILDER". This kind of Windows is perfect for what you are trying to do, because it comes with a wide array of drivers that will work with most situations: the kind where someone is trying to custom-build their own PC/laptop.
I advise you to go to Walmart.com or another that you are comfortable with, and order it. You won't be able to get it in any stores, I'm pretty sure, because MS has an arrangement with all the stores to only sell Win8. You can order Win7 from them, but it is easier on the web.
I bought my copy from Walmart.com and it works great. I think this will solve your problem, but you will have to order it fast to get it tomorrow. God bless.