Yes it's your hard drive. In this case your pc is only as fast as your weakest link. The bandwidth throughout of your hard drive is choking your ability to scrub through footage or render. Just get an SSD and you will see a huge bump in performance.
Edit: A 5200rpm drive is not meant for rendering, only for storage or low data demanding applications. They're super cheap drives for a ton of storage but they will totally render your specs useless. I would not even recommend a 7200rpm drive for video editing either (unless you truly enjoy punishing yourself).