Someone asks you for help with a computer that hangs at odd times. You turn it on and it work for about 15 minutes and then the computer freezes and power down. What do you do first?

Sounds like a overheat problem.

Check the fans to make sure they are all working properly, free from dust and debris.

Could be bad capacitors on the mainboard, check to see if any are starting to expand or leak, this can also cause instability and odd problems with a computer.

Could also be a virus - but this should get you started