I'm Greg, a volunteer installation specialist and 8 year Windows MVP, here to help you.
Select the hard drive you want to replace yours and swap it in. You can probably find a video online for how to do this for your exact model PC.
I'd replace it with an SSD which is the best Upgrade one can have - lightning fast, faster than a new higher-end PC without one, especially if you do the gold standard Clean Install in this link which compiles the best possible install of Windows which will stay that way as long as you stick with the tools and methods given, has zero reported problems, and is better than any amount of money could buy: http://answers.microsoft.com/en-us/windows/wiki...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.