I have a Gateway cpu around 5 years old with Windows XP that has recently began shutting itself off 3-5 minutes after startup. Does not shut down, it shuts off, complete power loss except for the green power light. No error codes on restart. I run Norton Internet Security every Sunday morning. Sometimes I can just push the power button and restart, sometimes I have to hit it 3-4 times. I have blown the dust out, ran chkdsc / f / 3 (that helped temporarily). Gateway says to reinstall XP. Another forum says maybe the power supply but it is so random that it doesn't make sense. Any other ideas?
Emily

1- You can try another power supply and see if this will help.
2- Check the CPU fan as your processor may be heating up (replacing the fan and adding some thermal grease may help).
3- Run a diagnostic utility from your hard drive manufacturer to make sure that your hard drive is running fine.
Most important of all, is to backup anything that is important to you on this system in case anything happens.

the start button problem may be the answer. could be intermittent and cause shut off. power supply is also a possibility but i'd check the power switch first. make sure all the dust is out of the cpu heat sink/fan like deep inside the heat sink. dino is right about cleaning the heat sink and cpu thermal paste/tape and reapplying thermal paste.