When Running windows and it crashes you will get a blue screen and it will automatically restart, often it will restart too fast for you to see the error message.
To disable auto restart on system failure, so you can see the error message.
1. Go to Start -> Control Panel -> System (Windows+Pause works, too)
2. Go to Advanced
3. Under the Startup and Recovery section, click Settings...
4. Under System Failure un-check "Automatically restart"
If no errors then
1. overheat - check for fans running and clean heatsink not blocked with dust etc.
2. Psu dying voltage drops (most likely on a 3yr old DEll (PSUs are notoriously underrated)