SONY vaio CS 21S/P automatically shuts down

when i turn on the laptop, it just shuts down just like when power is cut off. i tried to re-install the battery pack but it results the same. i removed the battery pack and used the AC adapter but same results. so it is not battery getting low power. it works fine for a while but then it shuts down again after a while..sometimes it works out fine, sometimes not. virus attack is unlikely causing the problem because i just scanned my laptop the night before it behaves that way. i hope i can get a solution on this.