Ok so my iMac I got 3 years ago just recently started running slower in the past couple months, nothing too bad though. However just yesterday it became unbearable to use. 10 min to start up, once it was on took forever to open up any files etc. So I did some research online and I decided my best option was to reinstall the OS with my startup disk. I did that and erased all my harddrive( nothing important on it.) after doing that it ran great like brand new. Today I turn it on, it continues to work great for a few hours then it freezes in me. I held down the power button to restart. Turned back on and it was stuck on the white apple startup screen. After 15 min I decided to power it off and let it rest a while. After resting it still won't start up. Any help would be greatly appreciated.