Cheers Si. It could well be that - the symptoms look the same. It happened last Friday, but it may just have been that I'd put off the update in between times. I hadn't switched off the auto-updates and it was nagging me that there was new stuff to install. I'd been telling it to wait, but then I left the machine on and unattended for a bit and heard it rebooting and now rebooting is pretty much ALL it will do.

Annoyingly I know I HAVE an untouched XP Home CD somewhere that I found the other week and went "huh, that could be useful sometime, I'll put it away safely"...