[rockaway]

1) first check for virus
2) if there is no virus then it is overheating.it can be possible that fan is not

working properly.
3) it may be caused by the problem of hardware in which a software is corrupted.
4)it may be problem of power .either with your main power line may have loose

connection.

[hanleyhansen]

Also your power supply may have problems and needs to be changed.

[micsom]

goto start>>run>>type "shutdown -a" (without the quotes)