The OP is getting suppressed Blue Screens of Death.
OP: please do this:
Start -> Control Panel -> System -> Advanced System Settings (upper left hand corner) -> under "Startup and Recovery," click the Settings button -> uncheck "Automatically Restart" -> Apply -> OK.
Try watching a video online again. This time, the machine won't reboot. You'll get the REAL Blue Screen of Death.
Write down EVERYTHING on the Blue Screen and post it here.