Sounds like a codec issue in part, but not sure why your system would restart. Perhaps a graphic card problem (out of date drivers.)

I'd recommend VLC - (click on link) _ (choose nearest mirror site) for previewing as it has it's own built-in codecs & is generally very reliable. To see it's abilities, see http://www.videolan.org/vlc/features.html

Check for updates for your graphic card drivers. OR/And for any other drivers for your computer. That's generally what causes auto-reboots. Are you getting the dreaded Blue Screen of Death (BSD) just before the computer reboots? If not, it could be a bad ram chip, or overheating. Perhaps an inefficient fan or inefficient heat-sink.