Video output on acid after 10.5.7 update to 20" iMac

Question

Greetings all,
one of my colleagues upgraded to 10.5.7 on his 20" iMac (with ATI Radeon HD 2400 XT video card) through Software Update, and received the following lovely surprise upon rebooting.
http://www.maths.adelaide.edu.au/paul.mccann/post10_57.jpg
The screen is constantly drawing as if it's on acid: redraws begin correctly, but quickly degenerate into something similar to what's pictured when you move the mouse at all (or do anything with the application). This sounds distinct from most of the resolution-based difficulties, or black screens that I've read about in the forums.
The behaviour has so far survived: reboots, different users being logged in, PRAM resets, application of the combo 10.5.7 updater over the top, removal of a second monitor (and further reboots etc).
Any suggestions?
Thanks for any help,
Paul

Answer 1

Hi again,
Hmm, yep it does sound a bit strange (but only because of what I hadn't related in my initial messages: the effect begins at the login screen, before there's even a sniff of the desktop picture on the scene). But I did change things just for completeness' sake; to no effect unfortunately.
Here's another picture to give a better appreciation of a typical view...
http://www.maths.adelaide.edu.au/paul.mccann/fish.jpg
Any bells being rung for anyone? (Besides the "Cat Transform", which has already been remarked on by several people around here!).
Cheers,
Paul