A PC that reboots by itself is usually crashing, not 'deciding' to restart — the reboot is the aftermath of a crash that Windows is masking. The goal is to find what is crashing it. The best clue is the pattern: does it happen during a specific task, after a set time, or completely at random?
Look for the crash
Open the Event Viewer (right-click Start → Event Viewer → Windows Logs → System). Look for red Error entries around the time of each reboot. A Kernel-Power (41) event means the PC lost power unexpectedly (crash, or power cut). A BugCheck or WHEA entry points to a driver or hardware fault. These codes tell you whether to chase software (driver) or hardware (power, heat, RAM).
If it restarts after a specific task
If it only happens when you run a game, a heavy app, or a specific program, the likely culprits are the graphics driver or overheating under load. Update or roll back the graphics driver (the manufacturer's app or Device Manager), and check the temperatures — a free tool like HWMonitor will show you. If the CPU or GPU hits 90–100°C and the PC drops, it's thermal: clean the dust out of the vents and ensure it has airflow.
If it restarts at a set time
A reboot on the hour, or after you've been idle, is often Windows Update or a power plan set to restart. Check Settings → Windows Update for an active update, and your power plan's sleep/restart settings. Disabling automatic restarts for updates can stop the loop temporarily.
If it's random
Random reboots with no pattern most often point at RAM or the power supply. Test the memory with Windows Memory Diagnostic (search for it, restart, let it run) or the more thorough MemTest86. A failing power supply is harder to diagnose — the tell is a PC that reboots or cuts out under load (gaming, video) but sits fine when idle. Replacing the supply is a relatively cheap fix on a desktop.
Safe Mode test
Boot into Safe Mode (Settings → Recovery → Advanced startup → Troubleshoot → Startup settings → Safe Mode). If the reboots stop in Safe Mode, the cause is a driver or a program that only loads in normal mode. If it still reboots in Safe Mode, lean toward hardware (RAM, power, heat).
The pattern is the diagnosis: task-triggered → driver or heat. timed → update or power plan. random → RAM or power supply. Start there and you'll cut the search in half.