No input

[#1]

You need the keyboard to input, in order to get back into the bios to rectify the checksum error of course.
So I think I would strip everything out except the bare essentials, leave just the graphics output/ memory and cpu. Whilst doing this, check for anything that may be shorting out, then boot it and see if you can get the keyboard working. If you can't I would prob stop there and retire it.

[#2]

I assume you have tried other keyboards both ps/2 and usb? Are they full ps/2 ie not a usb with an adapter as sometimes they are dodgy.
Is there any cards you can take out in case they are effecting it?
Try unplugging it for a couple of hours.
Other keyboards
remove everything except HD.
swap memory
if all else fails hit it with a big hammer.

[#3]

Quote from: samuria

remove everything except HD.

Huh! 
Why leave the HD in?

[#4]

Nice one Peter!
I thought it might be a short circuit, but it never occurred to me that it would be due to vandalism.
Lil sh!*s