Thanks for the reply leafo. Sorry about the late response, I've been without a computer since I posted here. Turned out the video issue was a dead/dying video card.
TBH, I think the issue was my computer but it did give me cause to reflect.
Could a game crash a computer semi/permanently?
Re: alternate user; do you mean create a guest user that has no privileges? That sounds like a good start.
I think I might run any downloads in a window (rather than fullscreen) until I'm sure they're ok. That way it's at least easier to exit out of the program.
Is there a technical discussion thread that covers this sort of stuff already?