The computer shuts down

My computer (Inpiron 5100, Windows XP Professional) shuts down after (different) accidental ammount of time. This is not a virus (i've checked my machine with several anti-virus programs). the following data might be important for the evaluation:

- computer shuts down nearly always when I am watching films; (bu it shuts down on other occasions also).

- there is no warrning before it shuts down - it is just happening

- I have changed the place of my residence for Europe - so I am using different voltage now (on the other hand for sox months everything was allright).

Can anyone tell me what might be going on?