Random Re-boot - Advice please

English Hardware
1

I just had a re-boot. Never happened before. On my main box as spec below. Was playing video at the time (.mkv) in kaffeine.
Actually using kde3 build service.

Are there any logs I can check please. Hardware checks?
Thanks

2

Any clues in /home/~/.xsession-errors?

3

There is plenty in there, though mostly just flash player/FF npviewer.bin GTK widget errors.

Thanks for the clue though:\