Key Stuck - *EVEN WHEN NO KEYBOARD CONNECTED*

Question

Hello
The up cursor on the keyboard is stuck on.
But it stays stuck even when no keyboard is connected at all.
i.e. if you use the mouse to open a menu, the options will highlight from the bottom up.
If you type a document the cursor will keep flying back to the top.
It's impossible to enter any passwords as the invisible keystrokes of the up cursor seem to make it invalid.
It's exactly if something was resting on the keyboard.
I"ve tried different keyboards, including a BlueTooth one, so that rules out the connections.
And As I say it persists even when the keyboard and mouse are entirely disconnected.
It is still on when I startup from a bootable CD.
It feels as if some sort of hardware problem has occurred...
I've tried resetting the PRAM but as the keyboard doesn't function correctly, I can't use the startup shortcut.
I've unplugged mains power and removed the battery and pressed the PMU reset button, but to no avail.
I've started up from an OSX Leopard disk, but the problem persists.
This is driving me crazy, any ideas?

Answer 1

yes, I've tried with 2 different mice, and with no mouse connected.
Even restarted from an install CD with no keyboard or mouse connected at all, cursor is still jumping upwards, and the repeating chime that you get when the keyboard buffer is full (like when you acidentally rest something on a key) is constantly sounding.
It feels to me as if there is some false signal getting to the keyboard buffer that the computer is interpreting as a constant up arrow key press.
Any ideas? Is there a part of the internals of the computer that I should check hasn't come loose in some way, triggering this false key input?
The problem happened after I moved the computer from one part of the office to another.