I've had this a couple of times - PC decides to shut down during boot-up. Gets as far as the windows opening page and then just gets caught in a loop where it won't get any further.
Probably a virus. I've got Avira AntiVir antivirus and ZoneAlarm firewall, and I do a scan twice a week. I've also got Stinger for the real nasties - running it right now.
I dealt with it last time by opening in Safe Mode and doing a System Restore, taking it back a week or two. This time it didn't work, until I turned it off and let it settle for a minute or two. Fear I may have swept the problem under the carpet rather than solved.
I've still got odd remains of Norton rubbish on the PC which I need to get rid of (even though I've removed the application). That may be related.