I guess we have to pull everything out of your nose then and this thread will have many posts.
What videos?
Online or via a video player like QuickTime or VLC Player?
And again, does the Mac just shut off and then restart and log back into your account with no message window at all? Have you checked Console yet?
And do you know the Mac OS X version you run?
AND PLEASE, BEFORE YOU ANSWER ONLY ONE QUESTION AGAIN: