The Problem: I have a computer at home that started randomly and suddenly shutting down due shortly after logging into windows, or just waiting a while at the bios or the safe mode prompt.

The Research: Initial research told me that it is most likely something wrong with the heatsink and cpu fan unit. I checked and CPU was running at 85C. No wonder it was instantly powering off, it must have been a safety option to stop it from burning up.

The CPU is a Core 2 Duo LGA-775. I came across this law firms warning that LGA-775 Intel heat sinks have the tendency to come undone over time, and especially under frequent temperature changes (fits the setting for this room). I touched and pulled on the heat sink unit and it seemed to just rattle and move, which is bad.

The Solution: If you have the same problem, just re-seat the heat sink unit. Instructions with pictures can be found here. A quick clarification and important note on the instructions: Make sure the arrows on the pegs are facing away from the heat sink. After that, pushing the pegs in and hearing a click is all that’s needed. (Using a screwdriver after that to turn the pegs takes OFF the heat sink, it doesn’t tighten it.)