Taking a desktop PC as carry-on luggage

As a college student, I built a small form factor pc just so I could transport it as carry on when going home.

Even though it fit in the dimensions specified on the airline website, I not allowed to carry it on (they didn't even bother to measure it) and it didn't boot when it came out the other side (I found an "inspected by the TSA card inside the case and most of the components unplugged).

When travelling, I recommend taking something "normal" (I'm in robotics now, and they don't bat an eye at a bunch of tiny robots and tablet-like devices in my bag).