So, today, I decided to reapply thermal paste to a computer that is about 9 years old.

I unplugged everything. After I removed the heatsink, I cleaned both the heatsink and the cpu with rubbing alcohol. Put thermal paste on the CPU and put the heatsink back on.

I plug the power supply back into the computer. Right when I do this, the computer comes on with out me pushing the power button in the front. Nothing comes on the screen. The CPU fan comes on but the fan that is in the back does not.

What happened? Every time I plugged the power supply back in, it comes on.