Hi
I have been trying to work this out on my laptop, but it is vista, and I can't cause this problem to happen.
I've had a quick Google for you:
In XP, try Open Control Panel>Display>Settings tab>advanced, and see if you can find a rotate display option. The option should allow you to rotate the display back to its original orientation (hopefully)
Otherwise there may be a hot key that you accidently pressed , try control/alt and either the up or down arrows to see if this helps.(This doesn't work on my computer, but may be enabled on yours).
The other thing is that you could have some spyware or a virus on the computer (hopefully not)
Hopefully this helps
Nicola