Hi, I bought a used MacBook Air off eBay and it was working perfectly but I wanted to reinstall it. I did that and it worked but it was being incredibly laggy when transitioning between windows and scrolling. It was also using 8GB RAM nearly all the times and around 70% CPU. I then tried to formatted the hard drive and tried to reinstall it using the method where it downloads Mavericks from the internet. It went through all the installation but has been stuck of the Apple logo screen with the loading wheels spinning for over an hour. How can I fix this? Thanks!