Quote - Switching off on it's own can also sometimes be down to a driver, have you installed any new drivers lately?
Microsoft have an item on it in their knowledge base!

I've recently updated my graphics driver but this was after the problem with Poser switching off my computer as I too thought it may be a driver problem. Thank you for the suggestion, thefixer. I'll take a look at the Microsoft site.

Quote - If your computer is actually turning off (it happened to me earlier this week) it's an overheating issue, almost certainly. Hot summer days, plus a heck of a load put on the CPU during rendering and your CPU temps may be triggering an automatic power down (which most motherboards do as a safety to prevent your CPU from being fried).

These threads are discussing the same/similar issues:

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2702905
http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2702153

Thank you for the links, fiontar. It's interesting the turning off has happened to you too. As it's not that hot in the UK at the moment, I didn't think of overheating being the cause. Recently I've been using Poser at the end of the day after my computer has been on all day so maybe it is getting too hot. I will check this out.