multiple error messages and crashing on XP

I'm still having problems with this dimension 4600c after solving the login problem, I reinstalled XP, now, the computer will shut off without warning, then when it reboots itself, I get one of two error messages- caused by norton anti virus, or a device driver is the cause.

I'm still a rookie to computer troubleshooting, so any help would be gratefully appreciated. 

Thanks,

Dave