Login problem after installing F 17

[Andrew McLean]

I have been using Fedora 17 for some months with few problems. My PC has 
an Nvidia GPU, and I have used both the Nouveau and Nvidia drivers at 
different times.
I just reinstalled from the DVD (because of a problem not relevant to 
this specific issue, I believe).
When running as Live User from the DVD, the display is OK.
After installation, I get the login screen as expected. At this point 
there is a Menu bar at the top of the window.
After I login with my password, the screen changes to the usual mottled 
green background, but there is no Menu bar. Before I reinstalled, the 
Menu bar was present and I could use the system as normal.
How can I do anything without a Menu bar ?  What might cause this 
problem ? I have reinstalled again and get the same result. I did notice 
an error message to do with the Noveau driver, but can't access the 
details without rebooting into text mode by tweaking Grub. But could a 
Nouveau problem cause this behaviour ?

What else can I try ?
TIA
AM