You may be infected with a virus/trojan of some sort, i know that sub7 trojans are capable of this but i am unsure if this is the problem in your case.

Turn the monitor upside down... Sorry :p

1. Might be a prank. Have a look at the running apps for anyone you don't recognize. I recall a joke program called "australia.exe" did this, maybe something like it is being launched at start-up.

2. Some graphics chips allow you to rotate the display. Check the control panel for the graphics card [Control Panel > Display Settings > Advanced] and disable rotation if it's an option.