Vaio automatic shutdown

My Vaio VGN-CS26G automatically shuts down and it does it in a pattern i noticed. It shuts down first around two minutes after booting windows. Next when i boot it again, it shuts down around the windows loading time. it takes me about 2-3 times in order to get in to stay stable and on for a long time. I use my laptop without the charger attached (to save battery life?) and it works perfectly until the shutdowns occured. ( I think it also shutsdown with the battery attached and on AC power) There's nothing wrong with the power supply or the charger also. Could it possibly be a virus that needs to be removed by reformatting or just a hardware issue? *By the way, it's not an overheating issue because i just turned the laptop on then it shuts down after around 2 minutes