hi, no idea about the mouse.
I was going to suggest you install again, but when
comes the screen with this 10 or so boxes where you
can review the configuration, go on the video card driver
and select the vesa driver,
also start with 1024*768
You should be able to boot.
Once you have booted, click on the terminal icone (the black small screen)
look for any indication about the mouse
(scroll up down <shift><arrow up/down>
My guess is that the mouse problem is linked to the video driver
there was a problem at some point for the 6800 GT