OK, well, instead of scanning for malware, I formatted my laptop, which I've been meaining to do any way.
So with only the chipset, audio and video drivers installed, I installed Flash and tried YouTube on Internet Explorer instead of FireFox. Completely clean install, different browser... the bloody thing still crashes. Black screen, no activity, completely dead, have to force a shut down. What the hell?
At the moment, I removed the new Nvidia drivers (266.58) and installed the older 258.96 to see what happens. I'll report back once I have some results.
EDIT: Well, running the older 258.96 driver and YouTube plays video everytime without a problem. So, possible bug in the new driver? I shall have to stick to the older drive for now, unfortunately. Not ideal, but seem I have no choice.