If an old computer check the air vents for dust and fluff if so inside will also need cleaning. Could be the PSU on its way out. Try starting in safe mode[Just keep tapping F8 or F5 and see if it works on there.If so try last good configuration or system restore or look in device manager to see if there are any yellow exclamation marks beside any of the devices. Run your security programs also.
Hands-on: Samsung Galaxy S8 review