Not sure if this will help, but if your lid to the scanner does not close all the way, try placing a dark cloth over the top to create complete darkness around the lid area to prevent light from getting in.
I just got my HP 7410 scanner in today from ebay and I downloaded the drivers online, then scanned my first slab. It came out similar to yours. I then threw a black winter coat over the entire top of my HP 7410 and it prevented any light from the room entering in through the sides, since the lid doesn't shut all the way.
I scanned several times after that and got excellent scans. Hope this helps. Worth a try.