Ok, so my first Mac, and up until a few hours ago I was absolutely made up. Anyway, I clicked Log Off to see if my Airport would reconnect by itself - it's been being a bit picky about whether it automatically connects or not. The Mac didn't finish logging off, and just froze with the pinwheel. After a good long while I switched off the machine at the back and turned it back on again. I got a picture of a power button with "You need to restart your computer" or words to that affect, in various different languages, every time I turned the Mac on - just before it was about to prompt me to enter my password. So, slightly worried, I called Applecare. We spent an hour running various utilities on the system and holding down god-knows-how-many bizzare combinations of keys on the keyboard while booting up but everything we did still caused the error message to be displayed when booting up normally. I managed to go on Safe Boot with him and note down some things out of my Mail, then we initiated a clean reinstall after he spoke to some of his colleagues to get second/third opinions. OS X is reinstalling as I type this. I just hope it works when it's finished I've never, ever had a machine fail on me so quickly! Talk about unlucky.