|Time to boot to the xp cd and try a repair or maybe make a bartsPE cd and boot so you can view the event viewer logs. In either case if you can't boot to normal or safe you have to start some repair. It may be that on safe mode you can see where the system stops for a clue.|
Things that could also cause it would be hardware. Memory damaged, power supply bad, cpu getting too hot. Might boot to bios and see if any logs exist or any health settings can be viewed.
Why did it take me over a year to phone in a problem to ATT?