Repairing the hard drive connector cable
I frequently swapped out hard drives, so frequently, I forgot they were actually attached to a cable, so without thinking, pulled and ripped the thing apart.
I knew what I was doing, but even then, the exchange was very simple and only took me a few minutes.
Don't be too scared of working on the interior of your computer (though try not to be as clumsy as I). Follow the advice, particularly grounding yourself before working, but don't be too terrified about touching things in your computer.