Thanks for the tip David. I had thought of delving into the preferences to see whether I could turn autocompletion off, but it's a feature I rather like! In any case, after multiple crashes, Mathematica 10's kernel decided it had licensing 'issues' and I think had become corrupted. I had to uninstall it. I hope a speedy bugfix for Windows users is forthcoming, but for now, I'm back to version 9.1.
