I have accidentall pressed something which has turned my screen upside down. How do I right it?

Do you have any monitor control software running?

Try pressing and holding Ctrl+Alt along with the ↑ arrow key and see if that changes it.

Any graphics card installed? Because this maybe controlled by your graphics card's driver and in that case you would have to check in Display Properties \ Settings tab \ Advanced.

See here for more: http://www.xp-tips.com/upside-down-display.shtml