Vaio keeps restarting when I play certain videos

Thanks in advance for the help. I just purchased a Sony Vaio RZ54G with XP Media Center. When I try to play certain videos, AVIs I think, the video doesn't play, and the computer restarts. I have downloaded the latest DivX codec, and I am not sure what other codecs to use, but if I didn't have the propoer codec, I would not expect the system to crash and restart. I am at a loss. Please help, either here or email to mmalt01@yahoo.com. Thanks again.