Xubuntu video display completely screwed (was: Can't log in to Xubuntu 9.04 Jaunty)

[Dave M G]

Ubuntu users,

12 hours ago I upgraded my Panasonic laptop from Xubuntu 8.10 to 9.04. 
The laptop has been running Xubuntu since 8.04, and never had any 
troubles of any kind.

However, on Jaunty, the video display is completely unusable. At the 
login screen and after logging in, the desktop resolution is over-sized 
in such a way so that only the top left quarter or eighth fills the 
entire screen. Also, it's not just that the desktop is larger than the 
screen, it sort of doubles back on itself, so there's a kind of 
rasterized look, and different parts of the desktop overlap each other.

In short, it is all screwed up, and unusable.

I can kill the X session and do things from a command prompt, so that's 
how I'll have to solve this.

What can I do to diagnose and solve this display problem?

Thank you for any advice.

-- 
Dave M G