Re: XPS 420 CPU Switched Off by Itself.
Go to your troubleshooting manual. The first thing you need to do is disconnect any non-essential peripherals, everything except the mouse, keyboard and monitor.
Next, check to see if the diagnostic lights indicate a problem. Try booting to your diagnostics partition (F12) and run the Dell diagnostics. If you are unable to do that, try booting to your Dell drivers and utilities disc to run the diagnostics.
When you post back, please make sure you give me the details about exactly what you tried and what happened.